You will have to run some tests. Scaling on the client side so all the server has to do is put the images together is much easier on a cpu-based setup. But you will have to run some tests before the event to be sure. A GPU can help a lot, but not necessary as long as you are okay with the fast preset and you aren’t trying to scale down like 4*4k/60 streams to one 1080p stream. Each client could stream at 720p and you’d never know combined into a 4-up 1080p for example.