Cleaning validation/foreign particles

I see many discussion on ppm or ppb of chemicals.

However what is the current practices on foreign particles? and more specifically about metallic particles?

We love to use stainless steel however when we have rotation or whatever, we may produce some particles…

We put filter but is the sieve thin enought? If your product is a suspension what you do?

Do we have to search for 0 particle in our installation?

If not how much? What is the maximum acceptable size?

Thanks for your help