How many legendaries are there in white 2?
#494 - Victini (Wifi Event, many out there, or Liberty Island Event - it might be possible to AR it but I haven't tried and missed the original)
#638 - Cobalion (B/W Mistralton Cave, B2/W2 Route 13; in B/W must be found to unlock other two of trio)
#639 - Virizion (B/W Pinwheel Forest, B2/W2 Route 11)
#640 - Terrakion (B/W Victory Road, B2/W2 Route 22)
#641 - Tornadus (B/W wandering, B2/W2 Dream Radar xfer)
#642 - Thundurus (B/W wandering, B2/W2 Dream Radar xfer)
#643 - Reshiram (game legendary, B and W2)
#644 - Zekrom (game legendary, W and B2)
#645 - Landorus (B/W Abundant Shrine [requires Tornadus and Thundurus from those games], B2/W2 Dream Radar xfer)
#646 - Kyurem (Giant Chasm)
#647 - Keldeo (Events only, most recent a Lv.50 one - I have a nice one I can clone if you want it since you're new to the games, as well as a Lv.15 one)
#648 - Meloetta (current Event, Lv.50)
#649 - Genesect (N/A Event was Lv.15, I've cloned it so I have it on both games since it was only for B2/W2)
I've only listed Unova here; Latios/Latias, Uxie/Mesprit/Azelf/Cresselia and perhaps others also appear. I know Latios/Latias is Dreamyard, and the Uxie trio disperse from a cave off on Route 20; Cresselia appears at Marvelous Bridge. Not sure offhand of others.
EDIT: I forgot about the Regis, and Heatran is answered below as you asked separately.
Where are all the legendaries? I went to two places so far that said they used to have legendaries there.
See my notes above. Note that Zekrom and Reshiram in the proper games can't be missed; in B/W, you can catch it before battling N, while in B2/W2 it's a post-E4 event after N gives you its ball and sends you to Dragonspiral Tower, which is available only after the E4 in that game. Others are as noted for both games, and some cannot be found at all.
Note I've offered you a Keldeo. Send me a VM (not PM, VM) if you're interested.
Is there a duplicate cheat? Or do I get more master balls later?
You mean cloning. I use an Action Replay, and yes, there is. Otherwise, no. And if you mean for items...have a Pokémon hold it, then clone the Pokémon. Claim the items before saying goodbye to the clones.
What pokemon should I use my master ball on? I there a new or a new two available in white 2?
I never used mine in B/W. Still have some to catch in B2/W2. In SS I used it on Ho-oh because it was 15 levels above me. :D Never used another.
How many other games are avalable on the ds? What is 'Pokemon dungeon'? Is it something I should buy?
Gen. IV (D/P/P, HG/SS), Gen. V (B/W/B2/W2); Mystery Dungeon games are completely different in gameplay.
Should I buy all versions like white and black or now black 2 since I already have white 2?
This is your choice. Some buy both for PC storage space. Some only buy one because they're the same game. Some want to play the game again without losing all their Pokémon and thus use the second cartridge. Really your choice.
Is there a legendary in seaside cave? Or reversal mountain?
Heatran is in Reversal Mountain; bring the Magma Stone to a chamber deep inside (I believe that item is on Route 18 somewhere). Seaside Cave has only a Crustle as a special Pokémon, but it's in the storyline; after catching or defeating it (it's at Lv.42), you can access the Plasma Frigate one of the several times you get to access it.
Cyclone
