There is no documentation worth the name for numbers like this. That is the salient fact for claims like this. Nothing written matters as it is false: only logic and scientific assessment of possibilities can suffice. The kind of scientific assessment (and actual physical studies) Delbruck repeatedly made (and performed).
My memory of Delbruck's writings on ancient warfare is filled with him saying such numbers were clearly false and he addresses that several times. I do not have access to my copy anymore (can one lose any selection of four books more worth re-reading than his?) so I cannot look more deeply at what he says on the subject. The quoted material simply cannot be all he said as he repeatedly said more than characterizing such claims with "is supposed to have" and would surely have addressed the ridiculous idea of victory over such an army with total losses under 20 men.
And in the quote, he DOES "is supposed to have" the idea, and finishes out with saying the Romans might have had numerical superiority. In what document in history, even fantasy writings by Roman authors, did you ever hear of a Roman army somehow approaching, much less exceeding 500,000 men? Never. Ever.
By the way, a constant Delbruck theme is showing the Romans usually DID have numerical advantage to go along with troop quality, tactical usage quality (they were the first to use tactical reserves in their basic line formations, not to mention on the legion ("division") level), and usually a massive supply chain advantage. And even without directly pooh-pooh-ing the idea, he does say a firm "No" if you read the quote and have experience with his general commentary about Roman armies.
Separate from all of that, Mithridates was the fellow who famously poisoned honey to discommode a legion or two (out of three as I remember that story) to ensure he could defeat them. What king who can field 500,000+ men at some point in time, not necessarily the same campaign season, but can do it... what king with that kind of resources poisons honey along the road to debilitate any portion of a chasing army of 20,000?
A surprised king? Sure. But he was on the offensive before encountering them and busy retreating. He cannot possibly have had no idea of numbers available for a Roman response so he surely brought a reasonable to the task number of men, and then shaded his chances with the poisoning idea, winning big on that bet. If he had nearly a half million more men he could call up, even calling up a quarter of them once it turned out the Romans would respond and retreating to them would have meant no need to hope the poisoned honey would work.
Such claims flat do not bear up under any scrutiny not of the "ancient aliens/ancient advanced civilization" quality.
Note as well that sources are Roman at least in the main. Not only did they always exaggerate enemy (barbarian enemies at least, or at least very distant, exotic enemies - the same way Sasquatch stories are now reduced to Oregon forests and Canada (can't talk about them living around Philadelphia, not even fools would think they wouldn't have been noticed tens of thousands of times, at least)) and always minimized their own numbers by, at a minimum, mentioning only the ROMAN legions in their forces, completely not mentioning at all the fact they usually demanded legions on a one-to-one basis from any local part of the empire they were in. So... weirdly enough, men who won (so lived...) and desired political benefit from such victories lied freely. If Eisenhower could have, and needed to, to win his Presidency, he would have turned out to have beaten the Germans with 40 men and no help from the Soviets. Fact of human life.
That said, Mithridates ruled something like 60 years and was a thorn to the Romans for basically all that time. He beat more than one Roman army/general, and did that kind of thing for decades. He likely did command overly large armies during this time as the legions had a string of victories to be envied and more importantly, had the supply chain to do so for a much longer string of victories. They were not yet at the point of "if they're too good, they might turn on their emperor... me... so..." so beating them outside Italy meant real, meaningful victories. What Mithridates accomplished was real. But given the quality of the opponent, he almost surely did actually employ larger than we might think armies.
But not 500,000, and not with 15,000 men. Even when Caesar supposedly did something more impressive, he had to have built a containing wall holding the original enemy in place, and an outer wall to hold back half a million more Gauls. Rather than just retreating (like Rome, those walls weren't built in a day...). Sulla didn't even have to build one wall, just slaughter everyone at no cost and then do it again. Seems 20 years of hearing that story required some extra (two walls) thinking to sell it again when Caesar trotted it out.
Finding four million year old human ancestor bones suggests those two victories should have IMMENSE bone fields somewhere findable as they are only 2,000 years old. But who has announced such a find?
So NO, Mithridates never fielded an army that size.
Delbruck also repeatedly addressed such interesting topics as marching order and what kind of length such order placed upon the numbers claimed. Things like the roads they had to use and how that would have affected marching orders. And how even without a supply train reaching back to somewhere, their food, camping, cooking, and armorial supplies as well as literal "baggage" would have had to fit into the marching order as well. He compared them to "modern" Prussian (so not everyone bought into Imperial Germany in its early lifetime) marching order with, as I recall, a 100,000 man army (or so) stretching 90 miles. Ancient armies made do with less, except the Romans, and much would have been less bulky than 1880's-1890's Prussian equipment, but it seems a good bet it would have taken days to array for battle such a marching army.
(You read about chariot armies (too little documentation for Delbruck to even address) meeting and waiting 2-3-4 days before "deciding" upon battle. You hear stories that it was because they had to clear the field between them of stones and other things that could mess up chariots... talk about "leveling the playing field"... Who wants to bet against me saying read Delbruck and you'd think they were both really waiting for their troops to all reach the battlefield. And that the one who attacked thought they'd reached the greatest advantage they could in that regard? The tale of Mithridates' troops being defeated and then almost immediately another similar army was crushed, and at least the first one defeated at literally almost no cost fits a couple thoughts. The best is Sulla not pausing for a second, just tearing into a mess fetching up before him and wreaking havoc, driving it into flight back into the rest of the army in its marching order, suffering no real losses in the idea, then meeting the jammed up army with the driven elements pouring back into the rest and ending up on a field definitely not chosen to give advantage to their capabilities AND hugely disorganized... and again, ripping right into it, savaging it causing its complete rout and break-up. Lots of losses this time, normal ones probably, but lots, but conveniently left out of the story reported triumphantly in Rome. Maybe literally "triumph-antly"... maybe someone knows whether Sulla was granted a triumph for it?
Like so many of these stories, there is utter death reported for hundreds of thousands of these enemies. Conveniently, that meant the general did not have to account for the slave sale proceeds for those hundreds of thousands that surely would have been captured and sold as such if they had existed. Shares of such given to the soldiers (or... promised, anyway) would have set standards for generals for a hundred years. But nothing like that exists. No disturbance to economies in the areas and on paths back home as all those slaves entered them and gold and silver exited them. Just lies put to death in battle. They exist. They don't exist. And no one lost all those economic entities (slaves aren't armed and put into battle... citizens able to buy their own weapons and armor are, so "economic entities") with the utter shattering to their economy.
No evidence in favor at all. Mithridates accomplished what he did with reasonable numbers of men and it is all that much more impressive for being so.