I'm just sharing this information, since I couldn;t find any info on maximum number of channels in pd when I needed it. Back in april with panGenerator ( http://pangenerator.com/ ) we created an installation, in which we used four Motu 24ao audio interfaces, combined into a single composite device with soundflower on mac, to play back a total of 95 audio channels simultaneously (the 96th channel of last suondcard was left unused since we didn;t need it). It played very simple sinusoids, and the latency was set high (300 ms) but otherwise it worked flawlessy. So in case you ever wondered what is the limit of dac~ channels in a single pd instance - well, it's centrainly not less than 95. It might be even higher.
Here's the installation:
(the mirrors are actually attached to regular speakers which play back most of the time very low sinusoids - between 1 and 30 hz).