If the PSR-E463 works like my E433, then you can have up to 100 USR files on each flash drive. This number comes from the way the keyboard is designed. When saving or loading a USR file, you do not type the USR file name on some sort of virtual keyboard or anything like that -- nor can you type the USR file number on the numeric keyboard. You use the data wheel to select the name (or more precisely, the file number) of the USR file, and you can choose file numbers from 001 through 100. Therefore, there can be up to 100 unique USR files on any one flash drive.
When you save a USR file, the file will have all the data for the 32 registrations, the styles loaded in memory, and all the multi-track sequencer user songs stored in memory at the time that you store the USR file (I believe these keyboards can also store MIDI files in a separate section of memory, and if so, I don't believe they are saved or loaded when saving/loading USR files -- I believe they use separate operations -- but check this to make sure).
Again, I have the E433 -- the E463 has sampling data as well as data for tunable scales -- I do not know if this information is saved in a USR file for the E463, but I would guess that it is. And yes, whenever you load a USR file from a flash drive to the keyboard's memory, you will lose whatever registrations, loaded styles, and user songs (and possibly sampling and tunable scale data) that was already there, and it will be replaced by whatever is in the new USR file that you just loaded. So always make sure you back up anything important that you want to save.
As 100 USR files can be saved on a flash drive, you can easily set up different USR files to accommodate different genres of music or different songs.