Two ways, I can think of right away:
a) you can count up the weapons fired by checking ammo-count of the secondary banks of bombers. Which then can add to a variable, to which you can also add with the event that does the weapon-create. This as downside as it would not work for turrets unless you table in an ammo count for each turret.
Problem is then to confirm the destruction of a bomb before impact ... which can be managed with armor values and precise damage values.
b) Axem's Markbox-script has a handy collection of lua-sexp for the exact purpose:https://github.com/AxemP/AxemFS2Scripts/tree/master/MarkBox/data
with it you can count up the ammount of a specific secondary weapon fired in a mission (lua-mark-weapon & lua-count-weapon), and also run a distance check of a specific weapon type (lua-get-weapon-distance) to check for impacts.
b) is much simpler and probably exactly what you want.