Bigisle uses 5 per side... so 5 at least.
However... The more zones you add the more strat has to be assigned to each country. Strat facilities have a high object count and you'll start pushing the object limit if you don't watch it, it's fairly easy to breach the limit with only 3 zones if you add a combination of very few V fields, lots of large airfields and a couple of tank town things. This may change of course with AH2.
I've been where you are. It took 7 1/2 months to complete my terrain and I have a funny feeling I'm just too late to be able to get it in the MA. Shoot me an email if you want to pick my brain at any point.
Cheers.
Gatso
