Early reinforcements to Tunisia were a German armored division (10 Panzer) and an Italian (Superga) and infantry. Hitler promised more, and the retirement of Afrika Corps to Tunisia in February would add those forces to the mix, but these aren't relevant to Stalingrad Relief.
So it seems that in theory the Axis might have sent 1 German and 1 Italian armored division to partake in the relief, if they could have carried the forces to the far steppes in time, which is questionable. Certainly these forces would have had trouble with the difference in climate between the Mediterranean and USSR. It doesn't seem like this made a lot of difference here.
Also, historically the moves to Tunisia happened before Stalingrad was attacked. So you can't cast it as a choice. And if you don't send those forces to Tunisia, the Afrika corps is lost by January rather than May 1943.
Things were similarly upsetting for the Axis. Nehring, considered by most to be an excellent commander, had continually infuriated his superiors with his outspoken critiques. They decided to "replace" him by upgrading the command to an army and Colonel-General Hans-Jürgen von Arnim arrived in Tunis unannounced on 8 December to assume command of Fifth Panzer Army. The Army consisted of the composite Infantry Division von Broich/von Manteuffel in the Bizerte area, the 10th Panzer Division in the centre before Tunis, and the Italian Superga Division on the southern flank but Hitler, in an interview prior to von Arnim's departure for Tunis, had told him the army would grow to three mechanised and three motorised divisions. The Allies had made strong efforts to prevent the Axis build up, committing substantial air and sea forces to the task. However, Tunis and Bizerta were only 190 km (120 mi) from the ports and airfields of western Sicily, 290 km (180 mi) from Palermo and 480 km (300 mi) from Naples making it very difficult to intercept Axis transports which had the benefit of substantial air cover. From mid-November through January, 243,000 men and 856,000 tons of supplies and equipment arrived in Tunisia by sea and air.
The Relief Operation
By the time the German Relief operation got started, it was essentially a 2 Pz division attack. When the Soviets noticed this concentration, they diverted forces earmarked for a further offensive (Operation Saturn) to the area between Kotelnovko and Stalingrad. The offensive, now dubbed "Little Saturn" jumped off right around the time of this counterattack and destroyed the Italian 8th Army, threatening to cut off the relief forces and the forces in the Caucasus by taking Rostov.
When the Germans met this reinforcement, they were stopped cold and quickly the alarming breakthrough of Little Saturn led to an abandonment of the relief. Even with additional force near Stalingrad, this still would have been the case.
So what about Torch??
Torch doomed the Afrika Korps in Cyrenacia, no matter what the result of El Alamein. That's pretty good. It also enticed the Axis to send at least 250,000 additional forces to Tunisia, all of which were eventually captured in by May. It also gave a secure launching pad for the subsequent invasions of Sicily, Italy, and South France. That's a pretty good haul for one operation. It doesn't need Stalingrad added to it.