How many ARCs or Galley copies should be sent out for reviews? That depends on the book's topic and the publisher. How so?
Well, a non-fiction book on a particular topic will have options unique to its subject matter - specialized magazines and such. Some reviewers may not review certain genres of books either.
The large reviewers will not accept self-published titles, thus limiting the ARCs required as well. They might accept all larger publisher titles but not books from smaller, independent companies.
So how many should be sent?
Make a list of all the conceivable places you WANT to send a copy.
Then narrow that list down to the ones that will ACCEPT your book.
Then budget how many copies you can afford to send - the cost of your book plus postage. ($2.50 or so for media mail.)
Your budget may not quite match your review list! Just send to the best options on that list.
And realize that if you had sent a book to everyone on your first list, you would've wasted many books and spent a lot of money! Good reviews can really boost sales, but blindlessly sending out hundreds of books and really hurt the rest of your promotion budget!