There are 5. Entei, Suicune, Raikou, Ho-oh, and Lugia.
Use the master ball you get from profe elm
You catch them with a ultra ball
no.
you must die
5
it helps you go back in your game if you didn't catch a legendary Pokemon
No No
over 9000
yes you can also catch them in soul silver.
You can catch Lugia, and evolve Growlithe into Aracanine, which is not considered legendary, but literally has the title of Legendary Pokemon. Hope this helps!
In Pokemon Black Version the legendary is Reshiram and Pokemon White its Zerkrom.The only way to get both is to transfer Zerkrom to Pokemon Black Version or Reshiram to Pokemon White Version.
no it's not possible to catch rayquaza in silver but if it is a hacked version you can catch it