I've spoken with a lot of aspiring authors who struggle with the idea that they need to send their manuscript out to a lot of readers before they can officially call it "done." I'd like to explain a little more about the process and to dispel any myths, legends or beliefs that may be associated.
1. If you're embarrassed to have that many people look at your manuscript, you're not ready to be an author. Anywhere from two to eight readers will go through your manuscript once it hits the publisher's desk, and if it's published, with any luck you'll have hundreds and even thousands of readers taking a look at it. You need as much input as you can possibly get before it ever makes it that far. The publisher doesn't have time to sit and wait for the story to get good. It should be good from the get-go, and your readers can tell you if they're being captured by the story or not.
2. It absolutely does not matter how many times you read your work -- you will miss things. I can't emphasize that enough. You can pore over every sentence until your eyes are blurry, but you will miss things. You need fresh eyes to help you spot the weak areas. And even when you think you got something exactly write, you still got it wrong. Every single great author that has ever been discovered has made mistakes in their manuscripts, and they have all needed to be edited. There is no such thing as a perfect-the-first-time author. Although, the more you write, the more you will automatically write it better the first time. Not perfect, but better.
3. You won't only miss grammar and punctuation, you will miss plot points. You will neglect to explain things well enough. You will over-describe. You will use redundant words. You will say the same things over and over. You will be too repetitive. (I'll shut up now -- you catch my drift.) You'll leave out a crucial clue or you'll give it away too soon. This is all because you're human, not because you're a bad author. Your readers will help you catch these mistakes.
Even if you've paid an editor for a professional edit, it's still wise to have a few other people read it for you. The paid editor, while fabulous, may not have experience in economics, and your brother-in-law does and can help you out with chapter four. The paid editor may not speak Italian, but your uncle Herbert does, and can clear up some problems on page 300. Conversely, your mom doesn't know a dangling participle from a penguin, and the paid editor does, along with suggestions for how to fix plot holes and bad dialogue. You need to utilize whatever resources you have to make your edit as good as it can get. And if you still need convincing, scroll through my archives until you find "Why We Edit."