The amount of mana you spent to cast this creature is usually equal to its converted mana cost. However, you also include any additional costs you pay, including the cost imposed for casting your commander from the command zone.
If Jeleva enters the battlefield without being cast, then no mana was spent to cast it. This means X will be 0, so no cards will be exiled.
When casting an instant or sorcery card this way, ignore all timing restrictions based on the card’s type. Other timing restrictions, such as “Cast [this spell] only during combat,” must be followed.
If the card has in its mana cost, you must choose 0 as its value.
If Jeleva leaves the battlefield, the exiled cards will remain exiled indefinitely. If Jeleva enters the battlefield again, it won’t be associated with the cards the “other” Jeleva exiled. The new Jeleva will exile a new set of cards. Only those cards can be cast using Jeleva’s last ability.