First, you should probably try to assess how CPU and memory intensive the sites are. Their total number alone won't determine that. Powerful web servers can easily hold 1000 websites if they're small enough (in terms of either traffic or server resources usage, or both).

Second, you might never feel ready to put all your eggs in one basket, and who can blame you really? It might be that for you, it's best to keep on getting reseller acounts and have the sites spread on a relatively high number of servers. I'm pretty sure not everyone has the stomach to face having all of their customers' websites down simultaneously.