Why are these flights greyed out?

In the FMB under group properties for a selected flight I have say 'Flight 1' with 6 aircraft , 'Flight 2' with 6 aircraft , then flights 3 & 4 are greyed out. How do I have them not so that I could have a flight of 24 fighters in a group?

The same thing for German bombers , only seem to be able to have 9 bombers in a flight so I have to make more . It's really annoying.