You can have at most 25 Rune Pages for yourself at max. that is the highest number that you can buy from the League of Legends Store. However, this does not include the 5 preset Pages that you are provided with in the start of the game. That takes the total number to 30. That number is of 30 Rune Pages are rather adequate as on an average players only use 4 to 10 Pages. And a highly skilled player might just need one. Although you will be required to save ample RP or BE if you want to purchase a number of Rune Pages.