I don't think this gives the actual number of seats left, but rather the number of seats left for sale.

e.g.: One could have a flight showing Y5, with 20 seats open on the seatmap, yet already oversold (more tickets sold than seats on the flight). Of course that's a somewhat unlikely scenario.

If you care about how many tickets are available, then that info is available for free (a page with some links). I do not know how to find out how many are available if the number is higher than 9 (or lower for some airlines) except to buy a bunch until the availability drops below 9.