why do we read about Thom and Davram Bashere's  Mustaches?   Do they have 2 of them?

As far as I recall, 1 is the maximum number of mustaches on any character; unless each part of a mustache is counted as 1 mustache.

 

Davram's seems to be mentioned less frequently than Thom's.

I think it is just a different (older?) usage of the word, where each half (left side and right side) is a singular moustache, so that both halves together are moustaches (plural).  

@paul h,

that's true,usage of the word moustaches/mustaches(plural) is now archaic.