Short answer: we don’t know 🙂
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.