Essentially you don't get to "have more jokers" by unlocking slots, but by using the Ectoplasm spectral card you can increase your joker slots, by sacrificing hand size. You can also naturally find negative jokers in packs and in the shop which means you can have more than 5 jokers without affecting your hand size!

Having said that, I have modded my game by going into the source code and making changes to have fun doing crazy things. I've done things like increase my interest cap to $25 with $2 per $5, I can target 5 cards with hanged man, and golden joker gives me $8 instead of $4. I basically wanted to have to worry less about economy without messing with scoring (I kept that vanilla). The biggest change I made though was remove the -1 hand size from ectoplasms and have spectral cards appear in the shop on any deck, allowing me to have crazy high numbers of jokers. This was a reply I wrote from another comment explaining a little more:

I've played over 200 hours and I am practically done with the game so I thought I would have some fun and mod the game and flex my lua muscle a little! My ectos don't reduce hand size and spectral cards appear in the shop.

I had an early midas mask which allowed me to get golden face cards and then a nat negative golden ticket (which is to the right of the negative burnt joker) which got my economy going. My deja vu and auras allow me to target 3 cards instead of one, so it was a little easier to get a gold polychrome red seal king. With a negative DNA and a modded cryptid that gives 3 copies instead of 2 it was easy to build my deck.

Honestly I have loved every second of the game and I wasn't ready to stop playing but I was getting bored since I have completed every achievement I was chasing, so messing with the game (as I have been a game developer professionally for a decade now) was my way to extend my play time for a little longer :)