Would WebSockets be able to handle 1,000,000 concurrent connections?

Yes, but it’s expensive.

This question is not unique to WebSockets since WebSockets are fundamentally long-lived TCP sockets with a HTTP-like handshake and minimal framing for messages.

The real question is: could a single server handle 1,000,000 simultaneous socket connections and what server resources would this consume? The answer is complicated by several factors, but 1,000,000 simultaneous active socket connections is possible for a properly sized system (lots of CPU, RAM and fast networking) and with a tuned server system and optimized server software.