Newbie MTG question: Is X supposed to be Roman Numeral 10 or is it X like a variable?

[Soddda]

just started to play magic today so please bear with me

I have a card called "Protean Hydra" that costs X and a green and enters battlefield with x +1/+1 counters

Is X referring to the Roman Numeral 10 (So I need to pay 10 mana of any color I want plus 1 green mana and I put 10 counters on the hydra) or is X a variable like in Algebra where I can choose any number I want?

[Arix]

X is a variable. It means one of either two things - either you choose what it is (so you can pay however much you want for the Hydra and give it that many counters), or a placeholder for a number which is explained on the card (like Induce Despair).