Getting new spell slots in Elden Ring isn’t stat related, so you can’t force it to happen. Instead you need to go find Memory Stones that are scattered throughout the world.
The first one is pretty easy to get but some of them will require you to kill bosses that you aren’t ready for. Early on you will just have to deal with the fact you can only rock two spells for a bit. A pure magic user will need these more than a hybrid character, but both are going to want find these throughout the world. I also haven’t seen any spells that take multiple slots this time, but they might be later in the game.
There might also be a ring or amulet that you can get to get more slots, but I haven’t found one yet.
