Just run things and find out. It really depends on your network and environment. Take note when things act up or slow down and scale back.
I usually have over 1,100 torrents seeding but maybe a dozen or so are active at a time. Maybe more sometimes, I don’t watch it as closely as I used to.
A few times I’ve added hundreds of movies at once by importing IMDb lists and experienced network errors. I had to download them in batches.