It's too soon to worry. The quality of the commits so far is extremely high. Franklin & staff are going after the very top tier so naturally they're going to lose a few high profile people to Alabama, OhioSt, FSU etc.
If they get to December a little short of their numbers, they absolutely shouldn't reach for players just to fill out the roster. They'll have all kinds of options.
1) The new transfer rule is a huge deal and schools may actually want to save a couple of scholarships so they can take transfers. By next winter, PSU will be getting inquiries from dozens of players at 2nd tier schools.
Some of them are late bloomers who wanted to come to PSU but there wasn't a ship for them before, but they have since developed into great players and want to be on a bigger stage. Some are PA and NJ players who want to SEC schools and got homesick. Could be west coast players whose families since moved east.
Lots of reasons, but transfers are going to probably be a much bigger part of college football.
2) There will be some late bloomer 3 stars who, after fall football, are suddenly 4 stars, and if they get a PSU offer they'll flip.
3) Banking a couple of scholarships for 2020 is ok. The 2017 and 2018 classes, when put together with the existing 2019 commits, are well rounded and there is not a weakness. So it's possible the best thing will be to save a couple of extra ships to have a monster 2020 class, especially if there are more Pa. players.