I applied to six between January and mid-March, and I was mentally drained by the last one.
The reference works takes a lot of time. They basically write similar responses to your essay responses (4 or 5 questions requring a 200-1,000 word response). If you give them too many and they just do a find/replace on your essay and miss one, that application is killed. It's one thing for you to accidentally forgot to take Kellogg off an application to Fuqua, but if they do it, you're application looks way worse.
These are tough packages. Every schools makes them assuming they're the only school you're applying to. They want you to think through your application for weeks and months, not cross them off a to-do list of applications.
Also, remember a lot of these places charge $250 per application. If you do ten, and only get into one, you just pissed away $2,250 on nothing and have to come up with a $2,000 deposit in three weeks for school.