#define MAX_BROTHEL_ROOMS   3

new
    bool:gBrothel_RoomTaken[MAX_BROTHEL_ROOMS]
;

// In the command:
if(!strcmp(cmdtext, "/blabla", true)) {

    if(Brothel_GetFreeRoom() == -1) {

        return SendClientMessage(playerid, -1, "No free rooms.");
    }
}

// Check if a room is free:
Brothel_GetFreeRoom() {

    for(new i = MAX_BROTHER_ROOMS; --i >= 0;) {

        if(!gBrother_RoomTaken[i]) {

            return i;
        }
    }
    return -1;
}