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You can't stack multiples of the same bonus type; you can only ever have the single best one at a time. But you have to read the tooltips carefully to figure out what type of bonus you're getting and whether it will stack. You're looking for keywords like "
deflection bonus," "
morale bonus," etc.
Some common early-game spells that grant AC:
"Mage Armor" grants a flat
armor bonus, which means it won't stack with actual worn armor, no (or with Bracers of Armor); it's designed for mages and others who wear little to no protection. There's also a dedicated mythic ability that gives it a further +1 AC per mythic rank, which is wild.
"Shield of Faith" gives a
deflection bonus, which stacks with your armor bonus but not with other deflection bonuses (for example, from a Ring of Protection). It's a good early buff on tanks, but quickly gets outstripped because of the abundance of magic items that give deflection bonuses.
"Barkskin" grants an
enhancement bonus to your natural armor, which is potentially confusing because this
does stack with a permanent source of natural armor like the passive Iceplant hex, but not with other spells or items that grant a similar enhancement bonus (for example, an Amulet of Natural Armor). Natural armor is better than worn armor because you don't lose it when caught flat-footed, though it doesn't help with touch attacks. This one remains useful on lots of characters if their amulet slot is taken (Camellia!).
"Shield" gives a
shield bonus (and blocks magic missiles entirely). This doesn't stack with an actual shield, but is otherwise pretty useful.
"Reduce Person" includes a
size bonus to AC because you're smaller and harder to hit. ("Enlarge Person" includes a size penalty.)
"Veil of Heaven," an early Paladin spell for Seelah, gives a
sacred bonus to AC, which is a rare bonus type that will stack with everything except other sacred bonuses. The evil version of this is a
profane bonus, which some spells and items grant.
Another bonus you'll encounter early is a
resistance bonus (usually from cloaks) that improves your saving throws. Again, resistance bonuses do not stack with other resistance bonuses.
