How to do a free API load (performance) test?

First of all, my English is not good, I apologize in advance.

I have an API service that I use. But I don't know how hard this is. For example, I don't know how many seconds it will respond to a "get" or "post" request from 5000 different users at the same time. Is there a way to test this for free? I would be glad if you help.