These are fare classes, not really connected with seat classes inside the aircraft. I think Flybe are now all one-class cabin anyway. And all that number is saying is how many seats you could book in each fare class at the moment.
Even on a 747 the maximum number that can be reached is 9. Flybe having smaller aircraft have obviously decided to work with a lesser number. And some fare classes of course are not offered on all flights. You can't tell by a 0 if a class is all sold or just not being marketed on the flight (in some cases not at the moment but it can change).
You just get a feel for it. If you've noticed that Flybe never go above 6, and there are lots of 6s here, I would say there's tons of availability on the flight. As it gets close to being full the cheaper fares may become more limited in number, eventually going to zero, but the more expensive fares may still stay at 6. If the whole lot are 0, then the flight is full !