MP1-5: 3 Passives
MP5-10: 4 Passives
Edit: Also I believe this could help with the itemization issue. For example:
If you need more resource regen at higher MP then use the 4th passive to do that and gear without the worry of needing more regen on your gear.
Edit 2: 4th passive could have reduced effect compared to if you placed the passive in one of the first 3 slots.
Edit 3: Glyphs such as those in WoW maybe an idea also. But can only be used on higher MP.