How many websites can I host on a Managed VPS?
With any Managed VPS above the 1 Core option, you can host as many websites as you like. You're in control of how many websites are assigned to the VPS and how resources are allocated to those sites.
How can I monitor my VPS resources usage?
You can check that your site(s) are getting the resources they need.
Head to My20i > Manage VPS > Options / Manage
From here you'll see your Managed VPS usage including SSD Disk Space, CPU and RAM.