One 12" fan inside would work fine.

From the sound of your idea with the pipe through garage and to the door you will be needing to do some calculations on your own. There is some math behind exhaust work that is very important. It deals with length of run and how many bends in your ducting. It all relates to the CFM fan you need, especially since it sounds like there may be a good amount of bends there.

http://boards.cannabis.com/growroom-...g-hot-air.html
All to do about fans/exhaust.

Also remember that PVC when heated outgasses very toxic chemicals and will harm your plant. Make sure theres no PVC inside your tent.