A question about dream readar?

Question

1: how many dream orbs do you neat if you want lugia end are there more legends to catch

Answer 1

Between 400-3000 you can catch Tornadus, Thunderous and Landorus. After that, you will be able to catch Dialga, Palkia, Giratina, Ho-oh and Lugia, but only if you have their respective game (Diamond, Pearl, Platium, Heartgold and Soulsilver respectively).
Source: http://bulbapedia.bulbagarden.net/wiki/Dream_Radar#Extensions

Answer 2

You don't need any dream orbs.
All you need to do is place the game in the slot and turn on dream radar.
Soul Silver - Lugia
Heart Gold - Ho-oh
Pearl - Palkia
Diamond - Dialga
Platinum - Garatina
Without having the game you cannot get them using dream radar though.