Hello Dennis,
There is no rule of thumb here.
I use the following sizing for my master-master environment and for 8 satellites:
Icinga Master-Cluster: 4x vCPU, 8GB RAM, 40GB HDD
Icinga Database Server: 4x vCPU, 8GB RAM, 50GB HDD
Icinga Grafana Server: 4x vCPU, 4GB Ram, 100GB HDD
Icinga Satellites: 2x vCPU, 4GB Ram, 20GB HDD
It is important to check the number of hosts and services. My environment has 338 hosts and 2832 services. I hope this information was of some help to you.
Best regards
David