vCenter & ROBO licensing question

I presently have ESXi Essentials, which gives me 3 hosts and 1 instance of vCenter which can manage a max of three hosts.  I have 4 sites, and I'll have about 8 hosts / 12 VMs deployed later this year. 

The way I read it, it seems like I can purchase one instance of vCenter Server Standard ($4,995) and one vSphere ROBO Standard ($3,000), and this will cover me as far as licensing.  True?

I want to manage all VMs (potentially 25) from vCenter, not just 3 like I am doing now.

And a follow up question: I run a VMware vMA on each host for communication with the local UPS battery backup.  Do the vMA's count as part of the 25 VMs?

Thanks!