On the cover it just shows Charizard and Blastoise. Please don't call me stupid as I have never played Red and Blue and I wasn't around for the making.
You can get 4 legendaries legitimately, Articuno, Moltres, Zapdos and Mewtwo
Serebii - Pokemon Red/ Blue legendaries
You can also abuse the Mew glitch to obtain Mew.
Bulbapedia Mew Glitch