I just go by when I have an opening and can fill it. I would rather have a few chaotic days then lose a family who really couldn't wait another week to start. It will always be crazy when a new child starts and upsets the group dynamics.

At the same time now that I have many years experience it is easier to make the changes and let them start as needed. When first starting out I would take it slow and do two at a time then add in another one about 3 weeks apart till you are full. Then you will get used to that group and able to change them out as needed.