Original Text (PHB1, page 286): Once per encounter, you can spend an action point. When you spend an action point, it’s gone, but you can earn more. Earning action points—You start with one action point. You gain 1 action point when you reach a milestone (page 259). After you take an extended rest (page 263), you lose any unspent action points, but you start fresh with one action point. Spending an action point is a free action. You can spend action points only during your turn, but never during a surprise round. Spending an action point gives you an extra action during your turn. You decide if the action is standard, move, or minor. After you spend an action point, you must take a short rest (page 263)
House Rules: Action Points can only be used on your turn, and can never be used during a surprise round. As a free action, an Action Point can be spent, granting the player an additional action of their choice (standard, minor, move). Only one Action Point may be spent per turn, but there is no limit to the number of Action Points that can be spent per encounter. Action Points are earned at every new character level and by reaching quest milestones and performing acts of dramatic heroism. Examples of heroism might include, but are not limited to; The destruction of a boss-level enemy; The completion of a quest; The defense of an ally; A creative or dramatic display of bravery or skill; A catastrophic success. The awarding of Action Points is at the sole discretion of the Storymaster.
There are no limit to the number of Action Points that can be collected.