counting cheats its 34 without cheats 22
giritina, palkia and dialga
use an action replay or gameshark to catch them all
Giratina is a Legandary Pokemon of the Sinnoh Region.It is said to be in the Distortion World.You can catch it normally in Pokemon Platinum,Pokemon Diamond, or Pokemon Pearl.To catch it in Heartgold or Soulsliver,You need a special Arceus.
use the Pokemon locator.
Rotom ,Shaymin ,Palkia ,Dialga ,Giratina
Tyranitar
You can get Giratina, Azelf, Mespirit, Uxie, Dialga, Palkia, Articuno, Zapdos, Moltres and Cressilia without the use of special events/cheats.
You can catch virizion, terrakion, cobalion, zeckrom, thundurus, landurus,kyurem, and celihidron
in unova, about 7 legandary pokemon. in total, about 28.
platinum:arceus leafgreen:mewtwo silver:lugia emerald:rayquaza
well in Pokemon diamond it can be possible to catch all of them but the easiest ones are dialga,shaymin,mesprit,and the other two lake Pokemon i cant remember the names.
buy the Pokemon platinum book