I understand how to count in standard and compound time (eg X/4 and X/8) but I can't get my head around time signatures like 15/16 or hypothetically 21/32.

Am I right in asuming that one would count a sixteen time signature in semi-quarter notes? eg 1e AND ah, 2 e AND ah etc or am I totally off track here?

UG, can you help me?
well, i'd assume it's all the same either way

for example,
15/16 - 1 e and ah 2 e and ah 3 e and ah 4 e and. 1 e and ah.....

21/32... seems unnecessary to me, but good luck lol
depends on the song and what notes grouped together

15/16 could just be 5 groups of 3...so count it like 5/4 but with a triplet feel
or it could be a really irregular grouping where you just have to get into the groove of it
it seems like you understand how to count the rhythms. the trouble is playing them right.