I know some brothers here might be happy with 20 episodes, but I think the majority can see that another 12 episodes over 5 years would be overextending the lifespan of a simple (but very good) porn game about an MC "following his heart" at university. Going past the point of viability is the death knell of many games, books, tv shows, and movies, etc. Porn games do best when they aren't trying to be epic sagas following year after year of a man's life (without serious time skips). 16 episodes (covering one university year) would be enough to tie things up nicely, but DPC has got to put a cap on the "bigger/better" every episode mentality. Six months is already stretching the limits of my patience.
I can't see it organically lasting 20 episodes, but that's not to say it couldn't be made to stretch that far. If he tried, I think you'd start to see the strain long before the end though - on the story and on player patience. Two main ways he could do it:
1) Expand and Repeat - more side girls get a backstory, more parties need to be planned, more hangouts/dates, more minigames etc. A lot of this could be fun and well-executed, but the implication is the resolution of key plot points keeps getting kicked down the road. And with 6 months and potentially more between episodes, it'd be a long long road (especially for Jill fans). You could easily see him experimenting with the format: like having a whole episode as a giant free roam on the campus, or an episode set entirely in D&G (perish the thought).
2) Resolve and Reinvent - pay off most of the key plot points by episode 12 but then introduce new characters, new drama etc. A high risk strategy because you're banking on people being as invested in the new characters and story as they were with the old ones. Hardly ever works and I doubt it's what he has in mind since he says he's had the whole story mapped out from the beginning. But...it would get him to 20 episodes and beyond, and hey, the demand may be there from some fans for that to happen, not to mention the enormous financial incentive to keep the show on the road.