Hi,
"It depends" is the answer here Depends on:
the number of concurrent users on your XO instance
the size/frequency/number of backup jobs you have
the number of hosts and VMs managed by XO
Frankly, for a small setup (less than 4/5 hosts) with one users and few backup jobs, the default 2vCPU/2GiB RAM is enough. Especially since we improved various process to happen in a dedicated worker