I'd be pretty surprised if they had a contract that literally worked off of "You can make as many chaps as you want as long as the number doesn't go up to 20" like this, with how exploitable that'd be. If there was really a limited contract here, I would assume it would work off of time or amount of actually-published work in a typical 20 chap run or w/e. But also this is a webcomic published on pixiv, which afaik just gets e-book releases otherwise, so I would be surprised at that anyway since it seems essentially self-published.
Feel like the more likely route is that the author realizes he doesn't actually know how he wants to progress the relationship and is stalling for time. Or, on the more hopeful route, I've seen manga do this before because the author got busy with an adaptation deal, either to an anime, or in this case since this is a webcomic, it could be him going for a physical tankoubon release for the first 1 or 2 volumes.