can't recall where I was discussing this with relay operators, tldr different relays will reject payloads above a certain size, and that anyway it makes no sense for a human to be following that many pubkeys. I don't think they'll block you 

 Strfry has a hardcoded filter size limit so that is the restriction you will encounter most commonly.

See discussion here: nostr:note1rpawm3yaaw3z74c43upfzpez48fy2w4vhqsqh6dxfmr0ac7pfgjs06jm4e