Theoretically you can have as many as 15, 3 with 2 slots (top 12 at worlds), plus 12 with 1 (12 others in top 24 rankings or season's best or world junior medallists moving up). This is some way off but perhaps not as far as people might think. For example there are 9 skaters (listed earlier) who look as if they're guaranteed assignments next year. However there are a further 2 juniors, Anastasiia Gubanova and Elizaveta Nugumanova who are on the season's best list and will not be seniors next year. However as they and other juniors move up (there were 10 juniors over 180 at this year's Russian Junior Nationals) you could soon end up with 12 to 15 in the various lists who are age eligible.
At this point things become a bit tricky. At 12 skaters you're not so bad, 6 with 2, and 6 with 1, but at 15 it's a bit of a joke and you would end up with 3 with 2 slots and 12 with 1 meaning that other countries' skaters with 2 slots and lower positions could end up at the GPF ahead of Russians who only had 1 slot but placed higher.
Hence as mentioned earlier it could be an idea to increase the number of skaters allowed at each GP to 4. However this would mean 4 out of 12 were Russians which I can't see too many people being happy with. However 4 out of 15 or 18 would probably be acceptable, plus we would get to see skaters (of say 190 standard) who we would not get to see any other time, as getting to the major championships for these would be extremely difficult.
Anyway let's hope the ISU are aware of this and are planning accordingly. I can't see them increasing the GP finalists any time soon (though personally would love it) as this would detract from their major championships, but 15 or 18 (would like to see this for the men as well, pairs/dance seem fine) at an individual GP would be great. Only problem would be the extra time needed - ice resurface + extra group, but hopefully this would be manageable.