Zombie Apocalypse

When Zombie Apocalypse enters the battlefield, amass 2. Each turn, amass 2
{x}{t}: Amass {x}
{10}{t}:Amass 12
When Zombie Apocalypse has been on the battlefield for fourteen turns, remove it from play and amass {x}{x}, where {x} is how many +1/+1 counters are on your battlefield.