[Drebin]

pawn Код:

if(strcmp(cmd, "/plate", true) == 0)
    {
    new string[128];
    format(string, sizeof(string), "Your plate number is %d. Type /changeplate [8 chars] to change it.", PlayerInfo[playerid][pPlate]);
    SendClientMessage(playerid, COLOR_SYSTEM, string);
    return 1;
}

Try that.
Changed %s to %d