HW sizing

Small env

Monitoring up to 2k VMs, 10 storage devices, 50 databases

CPU: 6
Memory: 12GB
Disk: 500GB

Middle env

Monitoring up to 10k VMs, 30 storage devices, 200 databases

CPU: 12
Memory: 32GB
Disk: 800GB

Huge env

Monitoring 30k VMs, 100 storage devices, 1k databases

CPU: 24
Memory: 64GB
Disk: 1.5TB

The numbers are very rough estimates.
Start with these and then monitor the XorMon host via the self-monitoring feature to fine-tune the hardware resources.

There is no any limit in max number of monitored devices.