You may kick Urborg Lhurgoyf only once for its {U} cost and only once for its {B} cost. You can kick it twice by paying {U}{B}, but you can't kick it twice by paying {U}{U} or {B}{B}. You can't kick it more than twice.

Urborg Lhurgoyf's second ability isn't a triggered ability and doesn't use the stack. Players can't cast spells or take other actions after it enters the battlefield but before its controller mills cards.

If it was kicked, Urborg Lhurgoyf enters the battlefield with a power and toughness determined by the number of creature cards in your graveyard after you have milled cards for each time it was kicked. For example, if you have one creature card in your graveyard before it enters the battlefield and you mill two more creature cards with its ability, it enters the battlefield as a 3/4 creature.

The ability that defines Urborg Lhurgoyf's power and toughness works in all zones, not just on the battlefield.

The kicker ability doesn't let you pay a kicker cost more than once.

To determine a spell's total cost, start with the mana cost (or an alternative cost if another card's effect allows you to pay one instead), add any cost increases (such as kicker), then apply any cost reductions. The spell's mana value remains unchanged, no matter what the total cost to cast it was.

If you put a permanent with a kicker ability onto the battlefield without casting it, you can't kick it.

If a card or token enters as a copy of a permanent, the new permanent isn't kicked, even if the original was.

If you copy a kicked spell on the stack, the copy is also kicked. If the copied spell is a permanent spell, the token the copy of that spell becomes when it enters is also kicked.

If a spell's kicker cost was paid, the spell is "kicked."