i have 8 runes and a master yi meditativa in base ready, what hapens with the might with yi if i use alpha strike, i still can use the 8 might or it goes back to 4?

Yes — Yi, Meditative at Base still swings for 8 with Alpha Strike, as long as you still have 8+ runes when the spell resolves.

Alpha Strike only asks you to choose a friendly unit; the chosen unit doesn't need to be at a battlefield (the enemy units do). Yi's +4 is a continuous passive on his current Might (base 4 + 4 = 8), and Might is read off the board at the moment it matters, so the damage is 8, not 4 (CR 710, and 8+ means 8 or more — FAQ #5219). Exhausting runes to pay the 3 Energy doesn't remove them, so the count stays at 8.

The one real trap is the 1 Power. Getting Power from a basic rune means Recycling it (CR 164.2.b), which drops you to 7 runes — Yi becomes 4 Might and Alpha Strike only deals 4. So pay the Power from a source that isn't one of those 8 runes (or accept the smaller hit).