In my opinion its highly based on what kind of player you are. If your looking to become an expert on a champion, if you want to be more of a generalist or if you just want to play whats new or op. If your play style if more of being a specialist at certain champions then buy more pages. This will allow you to tailor your pages for certain match ups that will give you the edge to really take advantage of lesser experienced players. If your feel your more of a generalist then get more champions that you can experiment with. If you just want the flavor of the month wait and see what champions are the strongest then just pick those up with your ip.
I would suggest getting more pages from you op. It will help you tailor pages to specific match ups for your main champions.