There are 721 pokémon species in total, but only a few of them are considered Legendary or Mythical. There are 39 Legendary Pokémon and 12 or 13 Mythical. The Mythical amount depends on whether you consider Phione a Mythical or not. This is unsure and there's contradiction between official sources about Phione's status.
you can catch as many legendary Pokemon but only once unless you have it on the computer
there are many legendary Pokemon in Pokemon heartgold, but the best is HO-OH
There are 6.
The amount of available catchable legendary pokemon is three: Kyogre, Latias, Rayquaza.
7 "legendary" Pokemon
20
Just one
21
Moltres is the only legendary Pokemon in Mt. Silver for HeartGold / SoulSilver only.
There are 4 legendary Pokemon on Red, Yellow, and blue. Maltres, zapdos, articuno, and mewtwo.
It is a rare pokemon, but it is not a legendary pokemon
Unknown is not a legendary Pokemon but it is considered rare. Legendary Pokemon are one of a kinds.