Actually every client IP+port / server IP+port pair. Linux uses 60999 − 32768 for ephemeral ports so can support 28e3^2 = 784 million connections per IP pair.
Except your service is almost certainly listening on one non-ephemeral port.
But having "only" tens of thousands of connections per client is rarely a problem in practice, apart from some load testing scenarios (such as the experiment here, where they opened a number of ports so they could test a large number of connections with a single client machine).
But having "only" tens of thousands of connections per client is rarely a problem in practice, apart from some load testing scenarios (such as the experiment here, where they opened a number of ports so they could test a large number of connections with a single client machine).