1. How To Increase Attunement Slots Dark Souls 2
2. Attunement Slots 5e
3. How To Increase Attunement Slots Dark Souls 3

• By increasing the Attunement stat (at 14 or so you get an additional slot, and so on and so forth). Alternatively, there are two magic rings you can find that increase your attunement slots by 1.
• Dark Souls Wiki » Stats » Attunement Attunement is the stat determining one's number of attunement slots available for Pyromancies, Sorceries, and Miracles. Each attuned magic has a limited number of uses that can be restored by resting at a bonfire.

In-Game Description

Attribute governing number of spells that can be attuned. Boosts spell-casting speed.

Use

• Increases the number of Attunement Slots available for Sorceries, Miracles, Pyromancies, and Hexes.
• Main stat for increasing Casting Speed. Its effect is double that of Intelligence or Faith.
• Increases Curse Resist alongside with Adaptability.
• Secondary stat for increasing Agility.
• Increases number of spell casts. The amount of the increase varies between spells.
• Slightly raises HP.

Leveling up attunement stat do it at certain points. 0-9 = 0 10-12 = 1 13-15 = 2 16-19 = 3. There's a table of it at ds2 wiki. You can raise slots also with certains rings you come across during.

Attunement Slots

Equipment which increase Attunement Slots

Equipment which increase Spell Casts

Equipment which restore Spell Casts

Note: Saint's Hood, Hexer's Hood and Northern Ritual Band all work in the same way. Spell uses are being multiplied by 1.1 (1.2/1.3 for the +1/+2 versions of the ring respectively) and then rounded up. For example, having a spell with only one use, the number will increase from 1 to 1.1, then be rounded up to 2, resulting in two spell uses.

If you combine one of the hoods with the Northern Ritual Band, the spell use percentage will stack additively (10% + 10% = 20%, or 30%/40% for the +1/+2 versions of the ring). This means that, upon using both the standard version of the ring and one of the hoods, a spell with one use will increase its number from 1 to 1.2, then be rounded up to 2, resulting in two spell uses. However, using both on a spell with six uses, it will increase from 6 to 7.2, then be rounded up to 8, resulting in eight spell uses.

How To Increase Attunement Slots Dark Souls 2

Spell Casts

Depending on the spell, it may start with 1, 2, 3, 4, 6, 8, 10, 15, 20, or 30 casts at base Attunement.
The table below shows how many casts become available with increased Attunement ('-' is used to indicate no increase from previous cast amount).

Attunement Slots 5e

Note - Spells that start with 2, 3, or 10 casts can increase by different amounts. While there are two spells (with 1 cast each) that does not increase in casts (you can use equipment to gain more casts of these though).

How To Increase Attunement Slots Dark Souls 3