​According to recent leaks, Pokémon Legends: Z-A is set to introduce 27 new Mega Evolutions across various regions. Please note that this information is based on unverified sources and may change upon official confirmation.​

Here’s the list

Kanto:
Mega Clefable
Mega Starmie
Mega Victreebell
Mega Dragonite

Johto:
Mega Meganium
Mega Feraligatr
Mega Skarmory

Sinnoh:
Mega Froslass

Unova:
Mega Emboar
Mega Scolipede
Mega Excadrill
Mega Eelektross
Mega Scrafty
Mega Chandelure

Kalos:
Mega Chesnaught
Mega Delphox
Mega Greninja
Mega Pyroar
Mega Malamar
Mega Barbaracle
Mega Dragalge
Mega Hawlucha
Mega Eternal Flower Floette
Mega Zygarde

Alola:
Mega Drampa

Galar:
Mega Falinks

none of this is yet to be confirmed so as always take it with a grain of salt.