Granted we can fill a server with 1,000s of accounts, but when do you feel that it is no longer manageable or productive to put accounts on a server (server specs aside)?
It all depends. I have one account with 17G of data and about 1G daily of transit. I have others with 10M of data and who do't do 1G in transit in a year. Surely number of accounts isn't the only metric involved in server loading.