Is there a limit per how many times one can select best answer to a question?

One at a time per question
But you can change it unlimited times I think

Best implies one

You should find out experimentally: post something nonsensical, then reply to your own post twice, and start switching "Best Answer" between both replies. My assumption would be that there is no "Best Answer" change counter.

You could gift or post a trophy for second best 😊