I just had the urge to go through the AGS games DB and catalogue how many series actually get beyond part 1. There are many great games in those "episode 1"s but too many of them get no further. Of course, some game series have lots of epsidoes, even though they are not planned as such. Oh, and a demo for "part 2" doesn't count.
So, there are 39 games in the DB described as "Episode/Part/Volume 1". Then there are 12 that are Part 2. And 3 that are part 3. There seems to be quite a tail off. Maybe those extra episodes will arrive some day, and of course I'm not counting some series like Princess Marian, Ben Jordan, Barn Runner, Lone Case and Maniac Mansion that did manage to get a lot of games in. Still, it is indicitive numbers even if they are flawed.