There is no easy solution for this, we normally leave that up to the customer. Getting an exact number from using data in Protect may not be possible. You could setup a scan of all the possible IP ranges (can add multiple IP Ranges to same Machine Group) on your network and then either perform a scan or use Test Existence. This should give you an idea what machines are live on your network and can be managed by Protect.
Going to Help > About will tell you how many Deployment Seats you are using right now.