Dark Souls is a game that takes a lot of strategy and planning to be successful. One of the most important aspects of this game is your attunement.
Your attunement is the number of slots that you have to use in order to use certain spells or abilities.
There are a total of 36 attunement slots in Dark Souls. You can get up to 12 slots by using the Estus Flask, and you can get an additional 8 slots by using an NPC spell vendor in Majula.
So, if you want to use all of your slots, you will need to either find a way to get more Estus Flasks or find an NPC spell vendor in Majula.