I think a lot of people credit jack with 6 kills in the server room. I think there were only 5. If you watch the scene closely you will notice some poor editing and continuity issues. For instance Tony Almeida and Bill Buchanan on separate instance kill the same person, from almost the same exact spot. Looking at the guy go down as a result of both shooters frame by frame you can see it’s the exact same clip being used twice. Jack later shoots the same guy at the end of the firefight.
Due to the darkness it’s really hard to see who Bauer is shooting but I believe there is one instance where he shoots a guy and later shoots the same guy.
The 6 potential kills

Hit #1

Hit #2

Jack Hit #3

Jack Hit #4

Jack Hit #5

Jack Hit #6

This is a shot of the guy(Hit #3) just before Jack shoots him for the first time.

Hit #3 B

This frame is taken 28 seconds later, when Jack shoots the guy for the second time(Hit #5). He looks the same to me and is standing in the same spot. So whether it's bad editing or the fact that he didn't die I think you can only give one kill for these two shots.

Hit #5 B