Figuring out the right number of threads will help you to get a higher VpM and a LpM level. You have to consider the performance of the VPS, the number of proxies, and the emails to pick the right number of threads that you can add on GSA SER.
If you have a decent performing VPS, you can add 10 threads per proxy without an issue. (For example, if you have 50 fresh proxies, you can add up to 500 threads to your projects.) But this number differs in your project settings, the performance of the setup, and the Bandwidth.
It’s best to tweak the number of threads around and figure out the best number for your project.