Gen I: Articuno, Zapdos, Moltres, Mewtwo and Mew ( 5 legendaries )
Gen II: Entei, Suicine, Raikou, Lugia, Ho-oh and Celebi ( 6 legendaries )
Gen III: Rayquaza, Groudon, Kyogre, Regirock, Regice, Registeel, Latias, Latios, Deoxys and Jirachi ( 10 legendaries )
Gen IV: Uxie, Azelf, Mesprit, Dialga, Palkia, Giratina, Cresselia, Darkrai, Heatran, Regigigas, Manaphy, Phione, Shaymin and Arceus ( 14 legendaries )
Gen V: Reshiram, Victini, Zekrom, Kyurem, Cobalion, Terrakion, Virizion, Keldeo, Tornadus, Thundurus, Landorus, Meloetta, Genesect ( 13 legendaries )
By this char, it seems that they introduced the most legendaries in Gen IV. Starting by Gen III, they obviously wanted to show even more legendaries. But having more than 10 legendaries.. is it really a must? I mean having 5 legendaries per region is really enough. I don't see why they have so many legendaries now.
Legendaries are losing their "rareness" now. That's only my opinion, seeing we got like 48 legendaries.
I hope in the later gens, they would add less legendaries. Anyways they'll end up having more legendaries than the first gen Pokemon.