So you ask x people to write a letter. They send it in. Can you read the letter on the AMCAS site? If one is less than what you were thinking, can you opt to send all but that one?
No, you cannot read the letters on the AMCAS site. You're supposed to get confidential letters of recommendation (though many professors will let you read the letter, just so you know what is being said about you).
However, you can pick and choose which letters you want to send to individual schools.