We actually cooked as many meals as possible in the provided packaging - no need to repackage for cooking purposes, but we did place the 'Mountain House' packages inside a gallon ziplock while they 'cooked' - MH packaging at Philmont usually does NOT have the zip top like the version you buy in the store - so we used the 'holder' ziplock as leak insurance and a seal to keep the dust out. (reused the 'holder' ziplock for as many meals as possible) When done the meal was divided after 'cooking' and one person ate out of the original bag and the other out of their own bowl.... less dishes to wash!!
If you are taking enough bags for each meal, don't forget the chicken salad lunch #3 that needs rehydrated (very good I might add!!), the apple dessert in dinner #5, and dinner #4 has 2 items to rehydrate - Mexican Beef and the refried beans (we dumped it all together in 1 ziplock to rehydrate)... so based on Mark's method above you are up to 15 bags PER PERSON (30 per FOOD PAIR or 180 bags for the crew).....that's alot of unecessary trash in my opinion.
You can get by with only 5 ziplocks PER FOOD PAIR (3 PER PERSON or 36 PER CREW) if you used the original packaging to rehydrate the entire meal... Trying to remember the packaging...All MH meals (Dinners #1, 5, 6, & 8, Breakfast #5 & Lunch #3) on the menu could be cooked in the original packaging and the #2 Wise dinner can be cooked in its original bag - you will definately need a bag for #3 Backpackers Kitchen Santa Fe Chicken and #10 Turkey & Stove Top (we mixed the turkey in with the boiling water for the stuffing and cooked it all together) - that only leaves the 3 Richmoor meals - #4, #7 & #9 and the dessert from #5 that may need a bag for cooking (we put all the dessert in a single gallon ziplock instead of each pair cooking... also, DON'T put in as much water as the package says... actually you need VERY little for this dessert otherwise you have apple soup!)
Seems like 1 or 2 of the Richmoor meals were packaged in bags that could be cooked in, but can't remember for sure or find pics to support that thought.
We preferred to cook the entire meal in the original container if possible for several reasons - to reduce waste/trash, reduce dishes to wash, reduce chance of spilling either while 'cooking' or while transferring to other container, reduce unnecessary items carried (8-12 extra ziplock bags per person) and meals tended to get better mixed/flavor and ingredients mixed in better if 'cooked' as a whole meal (one person didn't get all the pasta on top or the meat on the bottom) We did use a long handled spoon to mix all the bags.
To answer your question though, "how many freezer bags should my crew of 12 take if we are going to rehydrate half the food in the orignal bag"....21 bags PER FOOD PAIR (11 bags PER PERSON or 132 per crew).