There are 8 new megas.

Mega Meganium
Grass/Fairy
High Def, Sp. Def, and Sp. Atk
Symbiosis Ability

Mega Typhlosion
Fire/Psychic
High Sp. Atk, Sp. Def
Berserk Ability

Mega Feraligatr
Water/Dark
High Atk, Def, and Speed
Sheer Force Ability

Mega Donphan
Ground
High HP, Atk, Def
Sand Stream Ability

Mega Slowking
Water/Psychic
High Def, Sp. Atk, Sp. Def
Dazzling Ability

Mega Xatu
Psychic/Grass
High Sp. Atk, Speed
Zen Mode Ability

Primal Lugia
Psychic/Flying
High HP, Def, Sp. Def, Speed
Soul Heart Ability

Primal Ho-Oh
High HP, Atk, Sp. Atk, Sp. Def
Fire/Flying
Soul Heart Ability