We would like to use Amazon Connect. We have multiple clients and would like all calls routed to one agent queue.

One of the major advantage in a single instance approach is if your requirement to handle multiple clients using the same pool of agents. This will be needed, as a single agent cannot login to multiple Connect instances using a single CCP with unified agent status across all instances, not supported.

But if your agents are dedicated per client, different instances per client has a lot of advantages on dedicated storage and reporting data to not be combined. And easier reporting. However in your billing, if these are all in the same account, they will all show up as one line item in the bill.

to do separate billing, you will need to use different account (like subaccount) to get different billing for each instance with limitation mentioned above.

Also do consider region phone numbers if you serve globally as some countries phone number can only be serve in some particular region as well. From agent perspective, they can be anywhere as long as they meet the network requirement

You don't have to create a different instance for each client. Amazon Connect is able to scale to thousand of customers calls per day. In fact the default limit of Connect instances per account is 2 (see Amazon Connect quotas), although this limit can be raised is not the intended use case to have one instance per client.

By having multiple instances you will incur also in higher costs, as you will have to claim at least one phone number per instance (see Amazon Connect pricing).