If you have a 7x7 pen and won't be free ranging, then stick to 2 ducks. I'm not sure of the min space recommendations, but I think going by those, then you can probably fit more as it might be 10sq ft per duck. But if you have 5 ducks in a 7x7 pen all the time, there isn't much in there for them and you will have bored ducks and a mud mess once you add a water source for them.
If their house is 10x7, then you have more than enough space for 5 ducks in there.
My girls are in 500 sq ft when they are penned up during baby fox training season. I have now 4 ducks and 1 chicken and I hate having them confined to that size space and it is grass free from them (though there are a few weeds that survive their abuse). And that is with them getting an hour or two of supervised free range time.
