ADC for many piezos at once
You can multiplex a single ADC, with throughput reduced, of course. So if you have a 1MSPS ADC that settles in 1usec (probably available on-chip with some micros) you could sample 100 inputs at 5kHz. 

Sampling at 5kHz with a realistic analog anti-aliasing filter on each input you might be able to capture 1-1.5kHz fairly well (the theoretical limit is 2.5kHz). 

That may be an acceptable bandwidth for triggering musical sounds. To that you'd have to add processing time which will probably be limiting. You might do better to devote a processor to each input or somewhere in between, but you will have to profile the code you intend to run. 



You might also consider using a simple analog-input comparator (with input protection) for each piezo and generating a digital signal. Something like an LM339. It won't be able to find the peak of the signal, but neither will you using any algorithm in real time, since it's non-causal. You could add a one-shot to each comparator output to stretch the pulse giving you time to sample one at a time (or maybe 8 at a time).