Demonic Covenant's first ability will trigger once for each player you attack with one or more Demons.

The card types in Magic include artifact, battle, creature, enchantment, instant, kindred, land, planeswalker, and sorcery. Legendary, basic, and snow are supertypes, not card types; Horror and Room are subtypes, not card types.

Demonic Covenant's last ability will only cause you to sacrifice Demonic Covenant if both cards have the exact same combination of card types. Supertypes and subtypes are not taken into account.

Creatures that enter attacking are never declared as attackers, and as such, they won't cause Demonic Covenant's first ability to trigger.