As already told you need licensens for your Hosts (vSphere ESXi Standard) and vCenter Standard(unlimited Number of Hosts).
If this is your first buy you can go for a Bundle like Accelerator Kit Std. (6x vSphere ESXi Std. + vCenter Std) which gives a nice discount and additional 14x vSphere ESXi Std.). If not already done signed up for a VPP account before request the offer(but for sure your VMware partner will take care) as same as for a DealReg.
Question: How many CPU Sockets in total your 10 Hosts have?
Regards,
Joerg