Not sure.  But ProConnect Tax has Lacerte under the hood.  I would surmise that it can handle as many.

This is the last count someone had in Lacerte: https://accountants.intuit.com/community/lacerte-tax-discussions/discussion/is-there-a-limit-to-numb...

At 256, the software still ran, albeit slowly.  With ProConnect Tax running in the cloud, I'd hope it has more resources and computing power to crunch the numbers better than a desktop.