6-8 letters are reasonable
1) from government agency first
2) independent next
3) from different geographical parts of world
4) from diffeent geographical parts of USA
5) bigman over layman
6) no more than two letters from same institute or city
Just for general reference!! Do not take it for granted. You need to decide based on your particular case or particular situation.
Good luck
greetings from yjy_wang@yahoo.com