Is it best to stay with one proofreader?

If you are comfortable with the results of the first proofreader, you can keep them.  However, I would recommend two, but it is mainly a matter of personal preference, and budget (if you are writing a book and hiring professionals to help you).

It may depend on what you are writing.  I work in the printing industry and say the more eyes the better!  I have seen plenty of marketing and advertising materials, over the years, go from the copywriter, to the designer, to the press proofs getting approved, the paper printed, and finally someone in the bindery (my end of the business) finding a goof.  I have dubbed a gal in my shop Eagle Eye Smith for this very reason. She started to wire-o bind a college calendar and asked, "Shouldn't this start with August and not January?"  Sometimes it is the simplest thing that everyone misses.