We are unable to add new PATROL agents, we get the following self-monitoring event: "No. of instances is above 100% of max limit 250000 on Infrastructure Management Server."
What can we do to add more PATROL Agents?

Users would need to clean up the number of instances being used. We have the max limit which cannot be modified. Review the environment for instances which are no longer needed. If all are needed, stand up a new instance of TrueSight to allow for more monitoring.

If you can reduce the number of instances, here's a quick tip

What I would do is set the parameter low in ./pw/custom/conf/pronet.conf and allow it to purge it.
In ./pw/custom/conf/pronet.conf:
pronet.agentcontroller.mfd.instance.cleanuptime=1440
restart the server and it will purge anything over 1 day old.
But you will have to let it run for a day to do this
 
Then remove the line, restart the application and it's back to normal.
That's the easiest and cleanest way for a customer.