There are no SignalR performance counters to keep track of the total number of messages in SignalR message buffers.
There is, however, a performance counter that tracks the total number of message buffers SignalR currently has in use. This performance counter is called "Message Bus Topics Current".
SignalR has one "Message Bus", but the Message Bus can have many "Topics", each of which can buffer up to 1000 messages.
It's true that a Topic gets created for each client, but that's not all. A Topic also gets created for each Hub, group and user name.
1000 * "Message Bus Topics Current"
will give you an upper bound on the total count of messages currently stored in SignalR message buffers.