Personally, I try to stick to the "one a day" rule. However, I think if you have a costume that's just for a shoot or event, or perhaps you just want to wear it a while but not all day because it's too hot or too heavy, then it'd be managable to bring a costume to change out of for that day.
After all, if you only have one or two costumes and end up wearing a costume for more than one day it can get pretty smelly from all the running around and constant wear.
So, my vote: One a day is perfect, 4 costumes is just fine, 5... sounds a little hectic, but managable depending on the outfits. And 6 really does sound like it would hamper con-time with all the time devoted to changing.
Whenever I find I have a million cosplays I want to do and only one con to premiere them at I just remember: Cosplay = Money. XD;