depends...
do you want a "sub mixer" meaning:
do want a seperate mix going into a "main" mixer?
or do you just want to "mix" the drum levels...
i use a 16ch main mixer and mic the drums thru that...
i have a freind that "groups" drums in his main mixer...
and i know drummers who prefer thier own "sub mix"
if you want a "sub" mix, then (depending on how many mics you want to use)
sholdn't cost too much...
for my 5 pc kit i use:
kick
snare
H/H
tom (internal)
tom (internal)
floor tom (internal ) see peter chris' mics from the 70's stage show (Kiss)
and 2 over heads...
a min of 8 ...
leaving me 8 channles on my mixer
here's a few links...
http://www.guitarcenter.com/ART-MX821-8-Channel-Line-Mixer-105925887-i1519779.gc
http://www.guitarcenter.com/In-Store-Used-USED-BEHRINGER-EURORACK-MX-2642A-106193732-i1719622.gc
all that matters is the number of XLR's to run your mics...
GL