In my opinion it is possible but you have to use own script. You can use INSTANA API in order to solve this problem.
1. Use this endpoint /api/instana/usage/hosts/
You will get number of hosts APM and INFRA.
2. Next you can compare the number of host with your threshold.
3. If you cross the threshold, then send custom alarm/event by pushing information to this endpoint:
You can find more information how to use API on: https://instana.github.io/openapi/ and https://www.ibm.com/docs/en/instana-observability/current?topic=apis-host-agent-rest-api#curl-example
Example
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Lukas
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