You can’t have too many editors when you are getting ready to self publish. But the caution is that you can pay big bucks and still not get the job done. Big name publishers have experienced editors in each area.
What is “getting the job done?” I have found that someone who is a voracious, fast reader may give you great suggestions for fixing your plot but will miss the typo errors, like “in” for “on.” Spell check doesn’t highlight these, so the brain of a fast reader makes the correction without consciously recognizing it.
If you are working in an area that you are not completely familiar with, say quotes in Spanish, get a native speaker of the language to proof read.