Everyone is using 100% of the days for park days. We never do that. We always have a couple of days for the pool. We're going for 8 days this year and I only bought 5 day park passes.
2 MK, 1 EP, 1 AK and 1 DHS. The last couple of years we skipped DHS because our kids are young, but we're going this year so we can see Fantasmic and some other things.
I don't know if the OP visits regularly, but that would certainly weigh into the # of park days. If not frequent Disney visitors and you want to see as much as possible without running around, I would say you really need 2 days at each park except for DHS right now, where 1 would be sufficient. On the other hand, 8 days of park touring without breaks can wear people out.