How can you find how many seats are available for particular flights ?
When you buy the ticket you can sometimes choose the seating on the airline website. You can see there what seats are available to you - as they are unoccupied yet.
You need the confirmation code (letters) to do that.
If you don’t have a ticket on that flight… I don’t know.
Is this seats left or generally how much are available?
For the later, you could check what type of plane usually is used on the route/flight you are interested in, so you get a rough idea about the overall amount of seats, like 30 rows x 6 seats. However, some seats could be more expensive than others, like those close to emergency exits. I guess that really depends on the type of airline.
It would depend entirely on the airline.
I recently flew Etihad where I received six emails in the weeks prior, inviting me to bid for an empty seat next to me. I chose not to, but the seat was empty regardless.
This business model would collapse if I’d been able to see that the flight was only 70% full.
(of course they reserve the right to sell my ‘empty seat’ and refund the bid)
You could always do a dummy online booking - that is increase your passengers to the maximum you can book (Qantas is about six).That would give you some idea.