Oh, I missed this, but I am glad you found how it works. It took me some time too, as seen in the other thread.
Yes, you need to have at least 24 switches configured in order to assign any function to them, even if they are “virtual” (not physically connected to any GPIO pin - that is what “-1” means). The first four are the V4 encoder switches and then come the 20 V5 “buttons”. If you use(d) some real physical buttons on positions 5-24 before, they would duplicate the function of the corresponding V5 buttons at the same position (as configured in the V5 profile). So, if you wanted to use them in addition to those 20 buttons, you would need to move them to position 25 and higher. (The other option would be to reconfigure the whole V5 profile, but I guess that would be more tedious.)
I wondered whether all the 36 (was it?) buttons could not be just automatically available by default and pre-configured to the virtual “-1”, unless explicitly configured otherwise. @jofemodo mentioned that he pre-configured something like that for the “Dummies” profile, at least. Maybe there is more in the way to make it a general default that I have not noticed.