"Some say..." that there are multiple Stigs, and the whole mystery behind the character is just a means of preserving continuity if one of them isn't available.
It's no secret that Ben Collins is involved with Top Gear, because he's appeared on it before and his name is included in the H&S report following Hammond's accident in 2006.
"Some say..." that former F1 driver Jean Alesi is one of them (a close-up shot of the Stig's face through his helmet had very similar eyes and eyebrows apparently), but the Stig doesn't show too much F1 style driving so I'm doubtful. I mean, for all we know, it could just be Hammond in a racing suit.