It's a great question, unfortunately...there's no way to know the answer. :laugh:
It's like asking how much money will I need for retirement, I don't know, but I know the more i have....the better. :-D
It's better to have them and not need them, then need them and not have them. :)
That being said...
Do you plan on harvesting honey multiple times, or are you hoping to do one harvest. If you're going for one harvest, and conditions are right, and your bees boom....I could see you stacking on 5 or more supers on each hive. Not necessarily likely, but possible!
If you don't mind doing multiple extractions, have a couple of supers for each hive, when they finish capping one, pull it, extract it, and get it back on the hive. This could even be done with a few frames at a time. Personally, I think it would be a pain, but it can be done, and I'm sure some do it that way.
Another issue to consider....I know your wife says she's limiting you to 2 hives. But, if both hives are booming and you walk out and check them one day and find half of one of your hives sitting in a clump on a low hanging limb 20 feet from your hive because they've swarmed. What are you going to do? Are you really going to just let them fly away, or do you want to grab a hive box and get them hived. You have to have the equipment to deal with that....It will happen at some point.
You might decide to keep them, you might sell them, you might give them away, you might eventually combine them with a weak hive.....But you have to have the equipment to deal with them at that moment.