The Q0 has 21 keys and I would like to use 21 macros, However, I can only see 16 macros in the VIA app, why is that?

Sorry, this is the limitation of the VIA application. There are two ways to make it work with more than 16 macros since it's open source.

1. Edit the codes to make the Q0 work with more than 16 macros.
2. Use VIAL instead, since VIA right now can only support 16 macros. https://get.vial.today/