There is no rule about how many hosts you may have. However there are CERTAIN guidelines and its recommended to have hosts for the following 
Receive Host : Used for all in process receiving 
Processing Host : Used for all Orchestration 
ISOLATED Host : Used for all IIS receiving
Send Host : Used for sending of messages 
Tracking Host : Used for moving data from MessageBox to Tracking DATABASE 
There may be more then one for each of these jobs depending on the WORKLOAD and resource usage INCLUDING host settings etc.

There is no rule about how many hosts you may have. However there are certain guidelines and its recommended to have hosts for the following 
Receive Host : Used for all in process receiving 
Processing Host : Used for all Orchestration 
Isolated Host : Used for all IIS receiving
Send Host : Used for sending of messages 
Tracking Host : Used for moving data from MessageBox to Tracking database 
There may be more then one for each of these jobs depending on the workload and resource usage including host settings etc.