Looks fun! How many regional forms does it have?
Custom, none. Only custom Pokémon are the 3 megas shown in the trailer.
Otherwise, the following regionals forms are available in Hoenn's Safari Zone:
Alolan Forms: Rattata, Raticate, Raichu, Sandshrew, Sandslash, Vulpix, Ninetales, Meowth, Persian, Geodude, Graveler, Golem, Grimer, Muk, Exeggutor, Marowak
Galarian Forms: Ponyta, Rapidash, Slowpoke, Slowbro, Slowking, Weezing, Articuno, Zapdos, Moltres (these last 3 are not in the Safari Zone, but in special locations)
Hisuian Forms: Growlithe, Arcanine, Voltorb, Electrode, Typhlosion
Paldean Forms: Tauros Combat Breed, Tauros Blaze Breed, Tauros Aqua Breed