Came across some interesting reading,

What is the optimal size of a terrorist cell

https://globalguerrillas.typepad.com/globalguerrillas/2004/03/what_is_the_opt.html

I think 5 is the right answer also tactically.
1 leader and 2 teams of 2 each, also in the field you can form a v shape with a front man and 2 flanks