How many recommendation letters are enough? If I apply to a job, and it asks for 4 recommendation letters, is it good if I submit 5 or more letters? Why and why not?
In the US in math my understanding is:


For the NSF fellowship follows the rules to a T.
For other postdocs there's probably no harm in submitting an extra letter, but there's also rarely a compelling reason to do so.
For tenure track positions it is common, and not frowned upon, to submit 4 or 5 research letters instead of the requested 3 research letters.  If you have a compelling reason to submit more (for example, you work between two adjacent fields) then go ahead, but if you don't have a compelling reason you're also not going to be penalized for sticking with 3 research letters.  It's better to have all the letters be great than to have more letters.


There's a lot of discussion about this in the comments on this thread.