Hi,
actually the audio expander capelet only has 8 inputs, but piezos normally do not need a whole lot of what the audio capelet gives you ( amplification, AC-coupling, filtering).
You can usually hook up a piezo disc to one of the analog inputs using two 1M ohm resistors between gnd and Vcc. So if you need 25 inputs, you should instead go for the multiplexer capelet, which will give you up to 64 inputs by multiplexing the 8 analog inputs. In your case, you can use 32 channels 5kHz. Additionally, you will have to provide the 1M biasing resistors,
Capelets will be for sale along with the rest of the Bela boards later this summer. Subscribe to the newsletter at http://bela.io/#contact to be notified when the launch takes place.
Best,
Giulio