Upvote:-1
As others wrote, that data is not publicly available for good reasons.
You can try to game the system by trying to book a ticket for each flight, one hour or so before it leaves. If they are still available, you know that the flight isn't full. If you care, you can try to book 9 tickets, and if they are not available, work your way down to know how many are exactly still there.
[Note that it doesn't work to get to 18 with using two devices in parallel, as the same seats are offered to all people that try to book them - until they paid, and you don't want to pay.]
Upvote:7
As a rule, airlines guard details about how full their planes are ("load factor") jealously, so this data is not available to the public.
However, price is a pretty good proxy for how full the flight is, and this data is available on sites like Google Flights. So if you check and see that (say) the Wednesday afternoon flight is always the cheapest, odds are it's also the emptiest.
All that said, these days airlines are pretty good at packing their planes to the last seat, so the only way to be sure of getting an empty middle seat is to pay for it.