In general there's 3 type of Buff/Debuff so far.
Type 1. Status Effect Skill [ ussually has a party wide AoE, but some aren't like Belphegor's 7 ].You can recognize them from their Skill Icon.
Type 2. Attack Skill with extra buff/de-buff effect [ e.g D'art self heal/buff skills , Nergal "Smothering Smoke" ]
Type 3. This one is a special buff/debuff From "Summons" [ e.g Yamata no Orochi -def debuff ]
- Same type buff/debuff doesnt stack with each other.
for example, On current Mission you can stack up to 3 [-def debuff] to Garuda with D'art + Nergal + Yamata no Orochi. add Fafnir, Dragoon buff, Brynhilder buff and you could practically hit almost up to 200% DMG for 2 turns.
[ the actuall multiplier might be differs since I'm just calculating from Visual. 4800 DMG to 9200+ DMG full buffed/debuff]