[HY]

At the top of script:

pawn Код:

new Registered;

Where do you have register system add:

pawn Код:

Registered++;

Then you can check how many players registered.

pawn Код:

CMD:registered(playerid, params[])
{
    new string[3];
    format(string, sizeof(string), "%d", Registered);
    SendClientMessage(playerid, -1, string);
    return 1;
}

Of course, you can optimize it by savin' this in a file, etc.