After talking with Support, here is a response that was given for future reference:
There is no hard core limit per machine agents but may be we would hit the default max metric limit of an machine agent . That should not be problem if your controller has the capacity and machine has enough memory. We would recommend 500 URL per machine agent.
We would still recommend to spin up multiple machine agent rather than putting everything on one machine agent. I would recommend you to spin up one and test it performance one at a time. So in case of any issue we can work on it.