The value of X is determined as Laughing Jasper Flint’s last ability resolves.
Laughing Jasper Flint
Laughing Jasper Flint’s first ability applies to creatures you control but don’t own as they enter the battlefield. For example, if you control Laughing Jasper Flint and Vial Smasher, Gleeful Grenadier, having a creature an opponent owns that normally isn’t an outlaw enter the battlefield under your control will cause Vial Smasher’s ability to trigger.
A card, spell, or permanent is an outlaw if it has the Assassin, Mercenary, Pirate, Rogue, or Warlock creature type. It doesn’t matter if it has more than one of those creature types; as long as it has at least one, it’s an outlaw.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Ed Repka
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
