F in this context means "First Class", more specifically it is the "bucket" for the highest class of First Class ticket that JAL sells so if there is no availability in full priced F there will not be any availability for other lower selling or award classes of F tickets.

The number after F is the number of seats the airline is willing to sell in the F bucket. So F7 means JAL is willing to sell 7 First Class seats at the highest price they sell. F6 means they are willing to sell 6 First Class seats. Note, this does not necessarily correlate at all to how many award tickets they are willing to offer but historically, over the last few years, JAL has only been willing to release a maximum of 2 First Class awards on a flight with 8 seats in First Class (and the 777W that has First Class only has 8 seats and JL does not oversell F so you will see a max availability of F8) from initial opening of availability to a few months before departure. A few months before departure if F is not selling they may decide to release an extra seat (maybe two?) and very close in, 15-30 days or so from departure, they may release much more.

Given that F8 is the max availability, if you see F8 it means they have not sold any F on the flight so they MAY be willing to release 2 F awards on that flight far out. It varies. Some popular days of the week like Saturday they usually release none at first but midweek they have in the past usually released 2.

So:
F8 means no F sold. Possibly 2F awards will be released on the rotating release day.
F7 means 1 F seat sold. Possibly 1F awards will be released on the rotating release day.
F6 means 2 F seat sold. You probably will not see any more F awards on this flight until at least a few months before departure. Possibly much more within 0-30 days.

You can use different methods to check availability in the F selling class. I use Expert Flyer which will show you a list of all the selling classes and how many seats the airline is willing to sell in each class.

ETA:
ExpertFlyer has both free and pay tiers. I believe you need one of the pay tiers to see flight availability data. They have a 5 day free premium trial. There are other ways to get flight availability data and I believe some are free.

You can find articles describing the fare code which explain much better than I have here. Here is one:
https://thepointsguy.com/guide/airline-fare-classes/