There are 2 black jacks in a deck of 52. 4 jacks in each suit, deck of 52 has 2 red jacks and 2 black jacks.

An Ace with any 10, Jack, Queen, or King is a Blackjack.  If you have a Blackjack, the dealer pays you one-and-a-half times your bet  unless the dealer also has a Blackjack, in which case it's a push  and neither wins.