My flip answer is simple - How deep are your pockets?
Let me first say this: I hate to pay for edits, BUT, I realize that my work is not perfect and needs edits.
So, with that in mind, let me explain how I get edits done.
First, I write the novel or story. Then I (that is ME, myself and I - the three of us) go back over and edit as much as feasibly possible. I correct words, punctuation, spelling, line structure, repetitious words, weak verbs and whatever else I see.
Now I send it out to my select group of reader/editors. Each person in this collective serves a different purpose.
Person #1 really tears into my work and edits with a strong arm: going over my spelling, punctuation, word choices. She leaves a trail of red you won't believe.
Person #2 doesn't like to offend so he really reads it more for content, context, and continuity. If something doesn't make sense to him or he doesn't follow the flow, he lets me know.
Person #3 is one who is outside the genre of the story. I know that doesn't make sense but he reads the story and lets me know what works and doesn't work. It is a strange idea but I figure if somebody who normally doesn't read, let's say, science fiction, and is given a SF novel. There will be valid feedback.
Person #4 is one who I use from the start as I bounce ideas and plots off her and give her small vignettes to read as I write. She is very busy but can read a page or two without too much difficulty and give me feedback.
As you can see, each person actually does edits but the one really gets into the nitty-gritty while the others notate those glaring errors. Surprisingly, not everyone catches the same errors.
These are friends. They do this sort of for free. Okay, in return, I do edits for them and read for plot, continuity, etc.
Anyway, once I get back their edits, I go into my WIP and see what I can fix. Remember, this is YOUR story and therefore, any suggested fixes are your decision to make. So, I do adjustments then I take one last pass at the story. I read it aloud! Slowly. Enunciating each word. This has to be done so your ears can hear. I realize this sounds silly to new writers, but trust me, your mind reads more than it sees. I read a sentence four times and it wasn't until I read it aloud that I could hear just how trite and awkward it truly sounded. If it sounds good spoken aloud, it will read even better in the mind.
NOW I pay the big bucks to a professional editor. Yes, I do professional editing but I'm not an idiot. As they say, any man who attempts to defend himself, has a fool for a lawyer. The same holds true for editing. Therefore, I put my final work into the hands of another professional who will be seeing it for the first time with an unprejudiced mind.
I jokingly claim my editor buys her ink in 55 gallon barrels and uses it freely. The first time I used her, I swear that some of the returned pages had more red ink than black ink. Not so today, almost fifteen years later. Nonetheless, she does use a lot of red ink.
Now the important question. How much? This is something that you, as an author, must decide since only you know your financial situation. There are editors who charge by the hour, by the word, by the page, and several other methods. Evaluate each method against your particular piece. Dependent upon the method, the cost could work out to be anywhere from $.50 a page to $15.00 a page or $40.00 for a manuscript to $4000.00.
Be sure to check out the editor, read the reviews, ask for references, etc. etc. Remember, this is YOUR money. I had a friend pay over $1,000 for edits. When she got the manuscript back, there were a total of approximately 300 corrections over the 500 pages -- mostly spelling and punctuation fixes. The editor's comment: good story, nice plot - ready to go. She sent it out and the first publisher was nice enough to respond back with: Good story. Too many characters and locales. Seek professional editing service.
She joined a writing group where I was a member and I saw the manuscript for the first time after it had come back from the publisher. She had over 100 characters and 22 locales, names that rhymed or where spelled almost alike. When I read the story, I was totally lost and confused, unable to keep characters at bay or remember which location they were at as she bounced between the 20+ places.
My editor offers a dual editing special. A person can be edited once through for $X OR that person can choose what she likes to call "double" edit. She edits, returns, you fix, send back to her for 2nd edit which she does and returns to you. The best way to describe this method would be something like -- $2/page for once through edits or $3/page for double edit. You pay for the double edit up front at the start.
So, how much or how many edits does an author want? As many as feasibly possible. How many do they want to pay for? Zero, but reality is -- as many as necessary to get the manuscript to be a final product.
Your readers deserve a solid, final product.