This is a difficult question to answer because an awful lot depends on your own personal expectations of what you expect a proofreader to achieve.
Having said that, a good proofreader should find all mistakes in a proofed draft, no mistakes should be acceptable; otherwise, what are you paying them for?
On the other hand, in my experience (and in my opinion) proofreaders are asked to do too much. If you want someone to check your grammar, correct word use, sense and consistency etc., hire a copy-editor, that's not the job of a proofreader.
I accept that the dividing line between what a copy editor does and a proofreader does is blurred but it's important to understand the distinction between the two to get the best results from both disciplines.
If you ask a proofreader to take on part of the role of a copy editor as well as proofreader they will make mistakes.