Tower controller reported misreading the writing on the flight progress strip which resulted in a critical ground conflict.

Narrative

Fighters calling for taxi for departure to XXR one after the other. Aircraft X followed by Aircraft Z followed by Aircraft A and Aircraft B. Aircraft Y was on a visual approach to RWY XYR about 5 mi final Aircraft X in position for departure on XXR. I looked at the fighter strips as they taxied out to note the number in flight. It appeared that Aircraft X was a flight of 2; however; they we actually a flight of 4 and the way it was written kind of looked like a 2 with a slant. I cleared the fighters for take-off and switched to departure then I noticed more fighters to start to roll after the first 2 then I questioned how many were in the flight. Upon closer inspection it was a flight of 4. At this point Aircraft Y was over XXR and did not over fly any of the aircraft departing but it did split the flight. I'm unsure if the 3rd or 4th fighter delayed their departure because they observed the traffic.One possible solution is to verify the number in flight either by the fighters telling us on check in when they call for taxi or having the LC (Local Control) controller verifying.