How come there are so many Dragon Slayers?
There being so many Dragon Slayers despite it being a Lost Magic is an element central to Fairy Tail's plot. Dragon Slayer Magic is extremely rare since the only modern way to learn it is to embed Dragon Lacryma into the body as Laxus, Cobra and God Serena did. Natsu, Wendy, Gajeel, Rogue and Sting are completely different cases as they were taught by dragons themselves more than 400 years in the past and were later send through time to year 777 as part of The 400 Year Plan. Back in the past, Dragon Slaying magic was far more common.