From my experience, I felt like the number of available party members in, say, Final Fantasy X, which had 7, was too few, because you used pretty much all of them in pretty much every fight. Yes, you had a party of 3, but because of on-the-fly switching of reserves, you pretty much had everyone all the time. But, 7 was generally enough in Chrono Trigger - you could only switch out of combat, so some variety was maintained.
Final Fantasy 6, OTOH, with 14 plus some guests, felt roughly right. It was closing in on too many because of some shortages in synergy, but it was pretty much right. With a party of 3, 14 would have been far too many. With a party of 5, it would have been mostly fine, but maybe verging on too few.
Final Fantasy 4, as another, had only 11 total playable characters with a party of 5, but that didn't feel too bad because you really couldn't control your party composition.
So, as a general rule of thumb, you need anywhere from slightly more than double to slightly less than quadruple as many playable characters as you can have in your party at once. If you can freely switch your party around on the fly, go for more, if you can't, go for fewer. Of course, leveling has a lot to do with that, depending on whether your reserves gain XP even when they're not actively being used. (Personally, I would prefer reserves that gain XP even when not being used, but more available options, rather than a strong incentive to just always use the same set of dudes all the time.)