If you purchase an an air assault as an asset you just specify what turn and where on the table it appears, and it comes in automatically in the turn and location you select. You need one for each chopper you're sending in. Basically the same as an arty or air asset
You can also call them in on the 'fly', using a HQ or CO just like a normal order but then they're subject to deviation and not coming in that turn just like called arty/air, and you're at a penalty for each extra chopper.
E.g. I often use three groups each of a Mi-8, three Conscripts with RPGs and usually an ATGW team or Recoilless Rifle etc for seizing an objective/vital terrain. If I want to call them in one at a time using (say) my CO I roll vs. his CV for the first at CV9, then motivate the foot a couple of times once it's landed, then switch back to the next chopper at CV9, do the same, then the third.
Or if they go in en mass to swamp AA fire I can try to call in all three on CV9-2 - 7, and carry on from there!