My level 12, tier 2 mythic wizard expended three uses of mythic power on his turn. It broke down like so:
1. Expend one use of the mythic power to cast Mythic Haste as a standard action.
2. Use a move action to draw a Wand of Greater Invisibility.
3. Expend a second use of mythic power to get a another standard action via amazing initiative.
4. Use this extra action to cast Greater Invisibility via the wand.
5. Expend a third use of mythic power to activate Wild Arcana as a swift action, then cast some other nonsense.
6. Take the extra move action granted by Mythic Haste.
Is this all legal? Is there any limit on the number of uses of mythic power per turn? Would it make me a munchkin to get off a fourth spell via Contingency?
Use of mythic power, like spending a grit point or ki point,is not an action itself. Most of the time, a particular use of mythic power has an action type specified (e.g., Surge), or is part of another action (e.g., casting a mythic spell).
In the given example:
1. This use of mythic power is a non-action. The spell is a standard action.
2. Straightforward.
3. This use of mythic power is a free action as specified in amazing initiative.
4. Straightforward. You aren't even casting a spell, just triggering one. Note that you could not cast a spell with this action, as amazing initiative does not allow the action to be used as such. Wands, however, are fine.
5. Wild Arcana has been errata'd to be a standard action. So this one is a no-go.
6. Straightforward.
Contingency: Yes, provided you properly prepared the contingency ahead of time. (How is this munchkining?)