Hello @elvin y.
First of all you can follow the License Overview dashboard for this information and secondly you can use the below plugin.
What makes you say license overview is not accurate?
Licensed units is a volume of licenses available for consumption, while current usage shows what is currently consumed by agents. Licenses are not tied to specific agents, and once a license is no longer used (e.g. the process shuts down) the license is returned to the available pool to be used by any other agent requesting it (hence why fluctuations are expected).
Provided that all your agents are connected and licensed, the difference between licensed units and current usage will always tell you how many are spare.