It's probably more on the order of a thousand or so mainframe systems delivered per year, based on average lifespan of one of these systems and the ~10,000 or so existing mainframe customers.

Then you have to consider that each CPC drawer for a mainframe has 4 sockets. And there are multiple CPC drawers per maxed out system (I believe up to 4, so 16 sockets per mainframe). And some larger customers will have many mainframes setup in clusters for things like parallel sysplex and disaster recovery.

So probably more on the order of ~10's of thousands of these chips getting sold per year.

So definitely not high volume in CPU manufacturing terms. But it's not miniscule.