There is 28 of them, 26 letters ? and ! unknown. They are all equally rare. You will find them at the side of solaceon at the cave.
b is the most rarest unown letter. u can crack the 3 codes on the walls by getting a seal case at sunnyshine city seal mart then talk to the little boy in that house nere the ruins and show him an unown and he shall give u a letter seal and the letter is what the simbol means u can cheat with this sites pokedex! there is a fouth code: floors 2-7 unowns are f r i e n d that spells FRIEND!
26, and about as rare as a dragonair, since there is a lot of them but you can only get them 1 place