Usually, I'd think there'd be like roughly max 7/6/5, as there should be -
1 do-something-special-to-get-legendary (dunno what's they're called) such as Mew, Jirachi, Celebi etc.
Different Element Legendary Trio (that MAY relate to a main legendary), such as the birds, dogs and regis
1/2/3 main legendary/ies (might be part of storyline), such as Kyogre/Groudon/Lugia
Of course, it all depends on the version, such as Rayquaza wasn't actually considred to be a part-of-storyline Legendary in R/S, until Emerald when he was used to calm Groudon and Kyogre (with a nice little cutscene).
Hmm, i'm confusing myself, lol.