Short answer: we don’t know ūüôā

Long answer: Depends on a lot of factors like:

  • how many¬†connections¬†can your computer can handle
  • your computer’s bandwidth
  • your program/script some¬†are better than the others on resource management
  • the sites were you will use the proxies. If you make too many connections, the site will consider as an¬†abuse and the proxies will get banned

So you have experiment a bit by starting with a small number of connections/threads and increase until you will see a degradation of your performance, because if you use too many, you risk to saturate your resources and some connections will timeout.