SC2Mapster

With the launch effects the ammo units are known as the casters. The only way around this is to have a launch effect that does a Search Area effect (low radius) with a validator for Unit Type (for the host unit) that then applies a Create Persistent effect with the Persist Until Destroyed flag from the Caster to the target. This persistent effect then applies another Create Persistent effect from the source (the host unit) to the caster (the ammo unit) which can then be validated.

Then that can be done with a requirement just compare the number of ammo units completed at unit (like the carrier) compared to some function of the count behavior (your veterancy) completed at unit instead of a constant.

Using the Arm Interceptor requirement as a basis you could get this expression: CountUnit(Interceptor,QueuedOrBetterAtUnit) < {Carrier must have fewer than 8 Interceptors}(4 + ((2 + CountBehavior(CBehaviorVeterancy,CompleteOnlyAtUnit)) / 2)) which would give 5 interceptors at level 0 and +1 every two levels. In short 4 + [(2+level)/2].

Unless by lvl you mean a seperate unit it is not so simple to do with data since the ammo units are known as their own casters. Only way I can think of is to link them using a forwards and reverse peristent effects (the reverse can then be validated for levels of the host unit and modify the levels of the ammo units using a modify unit effect).