I assume that it means legendary + mythical Pokémon.
I might be a bit late but:
Gen 1 : Birds + Mewtwo + Mew = 5
Gen 2 : Beasts + Lugia + Ho-oh + Celebi = 6
Gen 3 : Titans + Eons + Weather + Jirachi + Deoxys = 10
Gen 4 : Lake Guardians + Creation trio + Cresselia + Darkrai + Shaymin + Manaphy + Heatran + Regigigas + Arceus = 13
Gen 5 : Swords of Justice + Forces of Nature + Tao + Meloetta + Genesect + Victini = 13
Gen 6 : Aura + Diancie + Hoopa + Volcanion = 6
Total before gen 7 : 53
Gen 7 is a mess because legendaries can evolve but we have :
- Guardian deities, Necrozma, Magearna, Marshadow, Zeraora : 8
- Cosmog evolves into Cosmoem, which evolves into either Solgaleo or Lunala : 4
- Sometimes Type:Null and Silvally are considered legendaries, but they are already in the game so I guess they don't count.
- Sometimes Meltan and Melmetal are considered gen 7, sometimes gen 8. I think they are officially gen 7 now, but since they can't be played in Ultra Sun/Ultra Moon, I don't expect them in Reborn.
Note: Phione should also count as legendary but is already in the game.