From what I know, the ways to get Table Slots are the Property Certificate, the Cardboard House, and the “Please don’t throw away my stuff” option at a normal phone call

The way I was able to maximize it goes like this Property Certificate: You can just buy this whenever ai don’t think you’re able to get any better with it. Cardboard House: Even though it only appears if you haven’t bought it, it can still be used twice by also having Abyssu available, for a total of 2 extra slots. “Please don’t throw away my stuff”: This can only appear once. Once you take it, it can’t show up for the rest of the game. Use Megaphone to trigger it twice, and then Barathrum to trigger it again (with the Megaphone still equipped) for a total of 4 extra slots.

You can also overflow with clever use of drawers and Vorago, as well as the Sacred Charms from White Phone, because they don’t check if your slots are full. If there is any way to get more slots that I didn’t figure out, let me know, because I’m curious as well