We went from two to three with no problem, then to five, then to 7, then to 9, one died (FIP), back to 8, and now up to 10!
We have 7 litter boxes and scoop them 3 times a day. (it's just routine after awhile and you don't even notice the extra work ) There are toys and little furry mice all over the house, we have 6 posts for scratching, 5 of them sleep with us and all are healthy, so no vet bills right now. They range from 9 mos to 5 years.
During the day while they are sleeping you would never know we had that many cats as they spread themselves all over the house. Feeding time is when you KNOW there are a lot! (and treat time)
It's amazing how fun they are and we love them to death! Yes we spend a fortune on wet food and litter, but totally worth it!
(and when we go on vacation I pay my sister to take care of them or if my DS25 stays home, he will take care of them) They are such stress reducers!
So enjoy your new addition!