There are 6.
if you want to cheat to get all legendary Pokemon on ruby you have to play on the computer
The Legendary Pokemon in Ruby are Kyogre, Groudon, Rayquaza, Deoxys, Jirachi, Latios, Latias, Regice, Regirock, and Registeel.
No.
There is many legendaries in sapphire but there is one legendary Pokemon what isn't in ruby and it is KYOGRE check out www.serebii.net
Ground
unusual u will get legendary Pokemon
you can find the legendary Pokemon regirock
groudon but in a updated version.
Have a ruby or another sapphire trade you a legendary!
If you flee from one of the Regis in Pokemon Ruby it will disappear from the game. This is the same for all Legendary Pokemon in all Pokemon games. The Roaming Legendary Pokemon are an exception as they can flee from battle without disappearing.
It's a cave, there's is a legendary Pokemon in it.