For the main AO:
Then go to this file: Invade_&_Annex_3_3_6.altis\Functions\AI\fn_mainAOSpawnHandler.sqf
That's the file that handles the units spawned by the AO. You can alter which and how many vehicles are spawned in here.
For the side missions:
Go to the folder: Invade_&_Annex_3_3_6.altis\Functions\Units
in here you'll find the following files:
fn_sideMissionEnemy.sqf
fn_smenemyeast.sqf
fn_smenemyeastintel.sqf
fn_smenemyeastrescuepilot.sqf
fn_smenemyurban.sqf
These files spawn in the units for side missions. fn_sideMissionEnemy.sqf is passed parameters to determine how many units are spawned, the others are a fixed amount. If you want to alter how many units are spawned in in a side mission i suggest first looking at the files for those side missions which can be found in this folder: Invade_&_Annex_3_3_6.altis\Missions\Side
If you've got any more questions don't hesitate to ask.