How many accounts do you place on each server?

No more then it can handle. We know this sounds extremely vague, but there's no way to feasibly say "we put XX sites on one server." Sites come in various shapes and sizes, it is entirely possible that a large corporate e-commerce website may require its own dedicated server due to the amount of traffic and load they place on it on a daily basis, whereas a server that hosts nothing but smaller static personal pages may be able to adequately host several hundred sites. There are simply too many variables in the entire process to say "X sites per server, that's the target." We constantly monitor the performance and load of each of our servers, making adjustments where we can to provide all of our clients with the best possible performance. When the time comes that a given server reaches a point where we feel it 'has enough on its hands,' then we simply obtain a new server.