IT didn't exist at the time, so technically the answer is zero, none.
If you mean companies that are currently IT companies, and created and sold equipment explicitly for the purpose of use in the holocaust, that'd exclude any company outside German occupied Europe, Spain, Italy, Sweden, and Switzerland, as there simply was no trade during the holocaust years with Germany from anywhere else.
Of German companies, no doubt many if not most supplied items that found their way into the bureacracies and organisations responsible. Therefore a list of German IT companies, filtered for date of establishment prior to 1945, would be a good start.
For the others it's much harder as there's no doubt no records of export to Germany during the war left, those would long ago have been destroyed because they'd be too embarrassing for the countries and companies involved to retain.
IBM and other American companies (and many around Europe) certainly traded with Germany before the war, and some of that equipment made its way into the SS, SA, Organisation Todt, Gestapo, etc. etc.. But that wasn't delivered for the purpose of being used as it ended up.
And if you count equipment captured by the Germans in their conquest of Europe, the list of companies gets even longer. Would you count the manufacturer of every train and truck used to transport people to the camps? Every tractor and farm implement used to grow the food that fed the local police forces and troops in the Netherlands, France, Denmark, and elsewhere? Every clothing manufacturer that created the uniforms for those people?
Quite obviously at some point you'd find that every company in every country that ever traded pre-1945 with any country that formed part of the Reich could be implicated. And quite a few of those (if they still exist) are today (or have today) IT companies. Think British Aerospace, Dassault, Boeing, Saab, the list goes on and on.