[AndySedeyn]

Create a global variable that you'll use to keep record on the amount of connections, increment the variable when a new player connects and reset it under OnGameModeInit (when the server [re]starts):

PHP код:

new playersJoined;
public OnGameModeInit() {
    playersJoined = 0;
    return true;
}
public OnPlayerConnect(playerid) {
    playersJoined ++;
    return true;
}
// Assuming you're using ZCMD
CMD:lastserverrestart(playerid) {
    new
        string[61];
    format(string, sizeof(string), "%i players have connected since the last server restart.", playersJoined);
    SendClientMessage(playerid, -1, string);
    return true;
} 


If that's not what you meant, then please elaborate.