Originally, he was going to add more floors, bring in a sister, and more, but he found that 8 main girls was about his limit, and at one point was not going to expand that until someone's story was finished (potentially adding more as some finished).
That said, now the last room will be filled by a new girl sometime in the near future, if his last timeline stays on target. Depending on how things play out, that could be within the next several updates. The timeline got pushed back some, as far as the intended dates were concerned, but that should not change the number of updates before her intended release too much.
On the other hand, any far future plans are subject to change, we will just have to see how things go. He could keep adding new girls as the older ones finish their stories, or he could eventually move on to a squeal, or any number of other things.