Gen 1:
Articuno, Zapdos, Moltres
Mewtwo
(Mew)
Total: Four attainable, one event-only, total five
Gen 2:
Raikou, Suicune, Entei
Ho-oh, Lugia
(Celebi)
Total: Five attainable, one event-only, total six
Gen 3:
Regirock, Regice, Registeel
Kyogre/Groudon, Rayquaza
Latios/Latias
(Jirachi, Deoxys)
Total: Six attainable per game, two more from the other game, two event-only, total 10
Gen 4:
Mesprit, Uxie, Azelf
Dialga/Palkia, Giritina
Heatran
Cresselia
Regigigas
(Manaphy/Phione, Shaymin, Darkrai, Arceus)
Total: Eight attainable per game, one more from other game, four event only, 13 total.
Gen 5:
Kobalion, Terrakion, Virizion
Tornadus, Thundurus, Landorus
Reshiram/Zekrom, Kyurem
(Keldeo, Meloetta, Genesect)
Total: Eight attainable per game, one more from other game, three event-only, twelve total
So there is definite legendary-creep, although Gen 5 is actually one down from Gen 4.
