Explosive Mana Orb

Whenever an opponent casts a spell, counter that spell unless the spell's cotroller pays {x}, where X is half the amount of opponents you have, rounded down. Then put X mana counters on Explosive Mana Orb.
At the beginning of your first main phase, if Explosive Mana Orb has ten or more mana counters on it, sacrifice it, then add {5} to your mana pool.