Well, 12 including equipment. It's easy to add more slots though, just create more "item#" variables.
The reason I use 9 in the demo (and in my new game) is because I can't fit anymore text on the screen due to the print layout I chose which contains a few line breaks. I'm also displaying equipment on the same card, so the total line count leaves no room for more than 9 items.
There's nothing to stop people from using a more efficient print layout however. I think you got around 20 items displayed at once in your inventory engine, right TD?
I kinda like having a lower inventory size myself though, as it's a little more realistic. I mean, weapons and suits of armor generally weigh allot, so carrying 9 Iron Broad Swords or Chain Cuirasses would be well over impossible!
Still though, in gameplay terms the space is definitely needed, as players
love hoarding healing potions!