Each key is 24 characters long.
Each character could be one of 36 things. 1-9 A-Z
36^24 = 2.2452257707354557240087211123793e+37 <-Giant Number
At a rough guess I would say about 3 Million people play Arma 2 OA. Thats 3 Million keys,
2.2452257707354557240087211123793e+37 / 3 Million = 7484085902451519080029070374597
So your chances of finding a key are 1 in 7484085902451519080029070374598 (to the nearest whole number.)
Good Luck (y)