Higher quality - Yes. Bigger -Yes. More than 4? Okay, 5 seems to be right to me for the whole season (including European and domestic cup games). More than that, I think, would be going overboard.
There are only so many guys of good/solid quality you can carry on the roster before you have issues with discontentment, lack of sharpness/form from lack of playing time, and stunted development of the younger players. Say, we picked up 5 quality guys this summer - and by that, I mean guys who would be close to the level of the incumbents (or even better), then the squad looks like this:
GK: Mignolet, Karius, Ward
LB: ???, Milner
RB: Clyne, Trent
CB: ???, Matip, Lovren, Gomez, Klavan
CM: ???, Henderson, Can
CM: Lallana, Wijnaldum, Grujic
AM/WF: ???, Coutinho, Mane
CF: ???, Firmino, Origi, Ings
Some of these guys cover multiple positions because of their versatility, so an injury, say, to Mane and Coutinho at the same time could be covered by the new AM/WF and the new CF, for example (or Firmino / Origi could be pulled out wide). Full backs aside, this staffing plan would almost provide (non-youth level) cover for 2 concurrent injuries at each position. That is already a very unfortunate situation. If you're so unlucky to get more, then, yes, some U23 guys would've to be drafted in.
In the setup above, the 18-man squad is pretty easy to determine (and still, we stunt Gomez's and Origi's growth, and Ings' diminishing value). If we're buying way more than 4 or 5 guys, we're going to struggle. Yes, I get that injuries happen and everybody will get their chance, but rampant injuries can also not happen for a period of time, especially earlier in the season. I'm not sure it's helpful for Klopp to have to deal with additional unnecessary distractions from overzealous staffing.
Besides, if I remember correctly, generally, clubs that are in the CL use a rotation of maybe 18 to 20 players in the squad for the season. Perhaps 22 tops.