I don't know if there's anything listing numbers across systems in a way that can be easily compared.

You can see how many people are actively playing BG3 on Steam here, though (you can also see recent updates regarding progress on Patch 5 if you scroll a little):

https://steamdb.info/app/1086940/depots/?branch=release_patch4

Currently: 130,936 In-Game

This doesn't tell you how it's split between PC/Mac/Linux, obviously, but it gives an idea of how many are actively playing on a computer OS and not PS5.