Primordial Hydra enters 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.
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.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.