How many machines required for the default sensible configuration?

[ulil albab]

On the cluster documentation said

"1. The default configuration is to run exactly one Coordinator and one DB-Server on each machine. This achieves the classical master/master setup, since there is a perfect symmetry between the different nodes, clients can equally well talk to any one of the Coordinators and all expose the same view to the data store. Agents can run on separate, less powerful machines"

What is the meaning of "Agents can run on separate, less powerful machines"? should we have 3 other machines for hosting the agents? or do we need only single machine and installed all the agent in the same machine?

Thank you