Rulings

Garruk Relentless's first ability is a state-triggered ability. It triggers once Garruk has two or fewer loyalty counters on him and it can't retrigger while that ability is on the stack.
You don't add or remove loyalty counters from Garruk Relentless when he transforms into Garruk, the Veil-Cursed. In most cases, he'll have one or two loyalty counters on him.
You can't activate a loyalty ability of Garruk Relentless and later that turn after he transforms activate a loyalty ability of Garruk, the Veil-Cursed.
The second ability of Garruk, the Veil-Cursed doesn't target a creature. However, when that ability resolves, you must sacrifice a creature if you control one.
The number of creature cards in your graveyard is counted when the third ability of Garruk, the Veil-Cursed resolves. Once the ability resolves, the bonus doesn't change if that number changes later in the turn.
Only creatures you control when the third ability of Garruk, the Veil-Cursed resolves will receive the bonus. Creatures that enter or that you gain control of later in the turn won't be affected.