Primordial Hydra enters the battlefield with X +1/+1 counters on it.
At the beginning of your upkeep, double the number of +1/+1 counters on Primordial Hydra.
Primordial Hydra has trample as long as it has ten or more +1/+1 counters on it.
9/22/2011 Consider only the number of +1/+1 counters on Primordial Hydra when determining if it has trample, not its power and toughness. For example, a Primordial Hydra with six +1/+1 counters on it that’s been the target of Titanic Growth (giving it +4/+4) would not have trample.