It's one VC per SRM and one SRM per VC, each with their own DB, so unfortunately you'd need a whole lot of VCs, SRMs, and DB instances for this situation.
Each site could have a single SSO instance to authenticate all of these environments, but if they can't consolidate VC management then they're pretty much stuck.
UNLESS.... they want to do a shared recovery model. In that case they could identify one of the SRM/VC pairs as the 'primary failover target' and have all the remote VC/SRM instances fail over to that one. But they're not saving on VC/SRM/DB instances really as each Source environment would still need a dedicated VC and thereby a dedicated SRM to pair with.