first what is your mean about : while ( letter[1] ) ...
leter[1] return char * to first block of array at index of 1 .
secondly
how to use letter in lettercount function ?!!! letter is one of local variable of main function not lettercount !!!
third what is ',' after letter at the main ?
Fourth if letter[1] exist while ( letter[1] ) be infinit loop !!
Fifth i < letter[1] is wrong of course in your algorithm which letter is char *letter[] !! of course we dont have this type in lettercount function i say this entirely!
and another thing :
you cant send all of "this is just an example of many A aa Aa" into just letter[1] !!! each of word insert into one of index of argv