Most gifts have a duration and last until it expires (generally a scene). So, could a werewolf stack gifts like Razor Claws and Luna's Armor?
There is no limit.
I've just read over the rules for Gifts in W20, and there does not seem to be any mention of a limit on the number of Gifts a werewolf can use at once. However, many Gifts require actions to activate, payments of points of Rage or Gnosis to activate, or other costs like taking a box of Lethal or Aggravated damage. As a result, having an extremely large number of Gifts active at once is likely to be difficult.