I will assume you mean edits in a thread in the forum and not edits to pixels in the gallery tho they are mostly the same.

As many as you need or want.

Whenever you need or have time. This is especially true when another person makes a point or has a solution for whatever you are asking.

Whenever you want BUT its better to have a long thread (no limit) regarding one pixel because others here will help but are artists with their own stuff to get done and not really wanting to keep up and follow others around. Generally speaking one thread per pixelart but some like a running thread for all their WIPs.

When you are done with it and have nothing else to improve or edit at that time.

Never. When you're done you're done. If you have things to learn or want to experiment with start a new piece.