For applying to math postdocs on mathjobs, does "N Reference Letters" include one for teaching?
In my experience serving on large numbers of pure mathematics search committees at a couple of different research universities, people who read the files are often only dimly aware of what the ad requests.  The vast majority of files will contain 4 research letters and 1 teaching letter.  Even if they only ask for 3 research letters, it is a poor idea to not include a 4th if you have one since most people will have 4.

Some people go overboard and submit a ton of research letters (I've seen 8-9 in a single file).  This can also cause problems: 1. usually when someone has that many letters they are not uniformly strong/informed, and 2. it looks weird, sort of like a violation of implicit social norms.  Probably you won't cause yourself problems if you go as high as 5, but don't exceed that.