multiple gateways on one server

I'm not sure that scenario makes sense. There can only be 2 situations that apply as you've tried to show:

1) A truly shared environment with one server servicing all customers from one environment. By definition (security model aside) you have one server servicing multiple customers who are all able to see all data. In that scenario you only have one server so only need one gateway. Even if you have multiple databases on that one server, one gateway can interrogate all of them.

2) A single environment hosting multiple VM's. Each customer uses the same core 'tin' but each has their own virtual server on that box. In that case, each customer has their own server so each server would have a gateway, regardless of how many data sources were on that server. Having a gateway in each VM is not an issue because each runs its own services.

The only scenario that complicates option 2 is if there is a need to have a gateway pulling data from each VM. Then the host server itself would probably have to have a gateway installed, and that is pointed to each VM as a distinct data source. Getting that to work depends on how you firewall each VM, not how many VM's you have on the box.

I find it hard to believe that scenario exists unless each customer is going to the hosting service and asking them to provide their reporting. I can’t see multiple distinct customers being happy about a hosting organisation having full access to all their combined data, but it’s feasible. Most hosts only provide environment and support without any free access to customer data, not least because of potential conflicts between customers.