SPLA<->KMS server(s) question

Hello,

I’d like to get help with the following question:
We are providing environment to customers thru SPLA (separate virtual servers for every customer at our datacenter, client computers at customers’ sites, site-to-site VPNs inbetween). What is the way of installing KMS server(s) in an environment with separate customers (companies); do we have to use one central KMS (with multiple hosts if needed) what all clients have access to, or have to use one KMS/client company? Or can we use any of these methods depending on the customer size?

thank you in advance
Balazs