Maximum number of block erupters raspberry pi can handle?
You can probably go up to the maximum of 127 devices if you really wanted (the USB2 limit), remembering though that the Ethernet adaptor is one, and each hub also counts in one. Word on the street also indicates a 49 Eruptor limit, though that's probably just a software limitation. 

The transfer speed will never be an issue. 

The CPU speed will never be an issue.