Hi Chahdok,
Yes, the maximum is 20 and yes, any custom voices used in the styles will also be stored in here - any user voices stored will not affect the number of styles - it will still always hold 20. You will not be able to see these custom voices but they are there and the styles will be able to use them.

The styles will stay there permanently unless you delete them or replace them with new ones - they will not be lost when the keyboard is switched off.
Best wishes,
Tony