I have a socket.io server with multiple namespace and i want count how many client are connected to the current server.

With socket.io v2 I count the connection with this snippet

let clientCount = 0;
io.on('connection', (client) => {
 clientCount++;
 client.on('disconnect', () => {
 clientCount--;
 });
});

it works for the V2.

Now, i have upgraded to v3 and socket.io has made a breaking change, from docs.:

No more implicit connection to the default namespace

This change impacts the users of the multiplexing feature (what we
call Namespace in Socket.IO).

In previous versions, a client would always connect to the default
namespace (/), even if it requested access to another namespace. This
meant that the middlewares registered for the default namespace were
triggered, which may be quite surprising.

// client-side
const socket = io("/admin");

// server-side
io.use((socket, next) => {
// not triggered anymore
});

io.on("connection", socket => {
// not triggered anymore
})

io.of("/admin").use((socket, next) => {
// triggered
});

Besides, we will now refer to the "main" namespace instead of the
"default" namespace.

How i can retrive the total number of client connected to my server with the V3?

A workaround is access to

let clientCount = io.engine.clientsCount;

but in typescript clientsCount in engine is marked as private property.