How many individuals exist for legendary pokemon species?

In Japan, they officially separate "Legendary Pokemon" and "Mystical Pokemon"

Legendary Pokemon - Pokemon that have very special background story, which is generally the myths and tales in each region. They generally gives the nature some influences. In the game, they plays important roles in the story, or having special encounter. They are prohibited to be used in Official Competitions. The Legendary Pokemon are usually only one in the world, though not always.
E.g. The legendary birds, The legendary beasts, The Regis, Groudon, Kyogre, Rayquaza, Dialga, Palkia, Giratina, Arceus, Zekrom, Reshiram, Kyurem, etc.

Mystical Pokemon - Pokemon that their existence itself is special. They may not have any special background story. In the game, you can access to these Pokemon only through special events in the campaigns of toy stores or event sites. Mystical Pokemon usually have more than one individuals in the world, but because not much people have seen them, finding one is already difficult enough.
E.g. Mew, Celebi, Latios, Latias, Shaymin, Victini, Manaphy, Phione, etc.

Also, it should note that there are some Pokemon float in the gray area between the category of Legendary Pokemon and Mystical Pokemon, such as Mewtwo, Deoxys, Darkrai, Cresselia, Heatran, etc. Except Mewtwo, do all the other Pokemon in this category have only one or more than one individual is debatable.