Crescita dell'Idra

If Hydra's Growth leaves the battlefield before its enters-the-battlefield triggered ability resolves, the creature it last enchanted before it left gets the +1/+1 counter.

To double the number of +1/+1 counters on a creature, put a number of +1/+1 counters on it equal to the number it already has. Other cards that interact with putting counters on it will interact with this effect accordingly.