No more than 2 magical effects (spells or items) can apply to the same destination. A destination is any single stat or value, such as AC, attack bonus, damage bonus, a specific Save Roll, a skill, an ability score, etc. All versions of the same thing count as the same destination. This limit also applies to area effects.

Spell and Magic Limitations

Within a destination, you can benefit from up to 2 modifiers, but only if each one is +2 or lower.

If any single modifier to that destination is +3 or higher, only that highest modifier applies and nothing else stacks with it for that destination.

This rule also applies to penalties, not just bonuses.

Examples of Stat Stacking

A spell +2 AC and an amulet +1 AC stack for a total of +3 AC. Two effects of +2 or lower to the same destination can stack.

A spell +3 AC and a ring +2 AC do not stack. You get only +3 AC total. The +3 effect suppresses the smaller one.

You cannot run a potion +2 AC, a spell +2 AC, magic armor +2 AC, and a magic sword +1 AC all to AC. That is 4 effects trying to reach the same destination. Only 2 may apply. If more than 2 effects would apply, the creature chooses which 2 apply. Max allowed is 2 magical effects per stat if none are +3 or higher.

A potion of +4 Constitution and a spell +3 Constitution do not give +7 Constitution. You take only +4 Constitution, since a modifier of +3 or higher blocks other stacking on that destination.