If you're counting on the Allied side, yes.
The UnitID for you to fly the aircraft will be its count starting from 1.
That said, if you are going to fly on the "enemy" side, you have to disregard this XXX.data.ini count and start the enemy side from 1 again.
Example: You have a MiG that in XXX.data.ini is [AirUnit024], but if it is the FIRST aircraft on the Enemy side, you should consider it as UnitID=1, like this:
"[IraqUnit001]
Squadron=No.1 FBS
ForceID=2
UnitID=24>>> Here you must put 1 instead of 24<<<<<<
StartDate=09/22/1980
DescFile=Campaign1Desc.txt
StartText=Campaign1Start.txt
AircraftType=Su-20B"
And so on consecutively.
For the enemy side you should always start counting from 0 in the order they appear, ignoring the AirUnit sequence they are in.