Generally, rune pages can be shared by multiple champions. There are times though where rune pages can be extremely effective when they are made for a specific champion.
Standard AD Page: AD Marks, Armor Seals, MR Glyphs, AD Quints/Lifesteal
Ezreal AD Page: AD Marks, Armor Seals, Mixing MR Glyphs with Mana Regen, AD Quints
Standard AD Jungler: AD Marks, Armor Seals, Scaling MR Glyphs, AD Quints
Nocturne Jungler: AS Marks, Armor Seals, Scaling MR, AS quints
Standard AP Tank Top: Hybrid Marks, Armor Seals, MR Glyphs, AD/AP Quints
Mundo Top: Hybrid Marks, Armor Seals, MR Glyphs, HP/5 Quints
Ryze Top: Hybrid Marks, Armor Seals, Mana Glyphs, Mana Quints
Notice that a the standard pages, multiple champions can use. Some champions work very well with specific rune pages though which gives them a nice advantage in lane.
Ezreal tends to be very poke heavy and can run out of mana fairly quickly if you poke hard. Adding some Mana regen in place of some MR can win you the lane by letting your extra poke whittle down the enemy forcing them to either play defensive or potentially die.
Many junglers like Pantheon or VI build straight AD for damage from abilities. Some junglers synergize very well with the new Feral Flare or do more damage overall the faster they attack and so instead of damage building lots of attack speed greatly increases their damage output.
Many top laners will have pretty similar builds. Some tanky ones like Malphite or Shen might run those standard pages I show up top. Other tanks like Mundo don't benefit from the same. Mundo tends to farm with his cleaver which constantly drains his HP. He needs the extra health regen quints to safely farm from afar to keep his HP up. Ryze gets a huge bonus from having extra mana to damage. Level 1, if you have all mana Glphys/Quints can get his level 1 spell damage to 109. This is pretty significant.
Basically, you don't NEED to have additional rune pages. You can get away with having 2-5 but if you are vs a similarly skilled player, your better runes could give you the edge to win the match and potentially snowball your lane. Currently, I run 19 rune pages for a variety of champions.