The answer is zero. You don't add files, you replace/overwrite them.
There are 8 sound banks available, with each bank having a slot for ignition, impact, lockup, shut down, swing, and idle (and maybe more that I cannot remember at the moment, but I think that's it). The maximum number of font files that can be on the board is 48 (provided I'm remembering the correct number of slots).
If you want to change one of the sound fonts, you replace/overwrite what was there previously, as opposed to just adding a new file to the board. Which is one of the reasons it is HIGHLY recommended that you make backup copies of your files BEFORE you overwrite them, just in case.
And technically it only comes with 7 sets of fonts pre-loaded, as the Mute font is simply an empty sound bank. There aren't any actual files for it.
Ahhhh, okay. Point incoming to you sir, thanks.
I'll have to watch the How To videos on the Obsidian software and get comfortable with it before I start messing around with it, but i'm sure making backup copies of the existing fonts and switching them out can't be that difficult.