This counts Action cards in play, including Action cards played this turn, Duration cards in play from previous turns, and Reserve cards (from Dominion: Adventures) called into play this turn.
Other Rules clarifications:
Since the attempt to gain 2VP (and thus, the check to see how many Actions are in play) has "when-gain" timing, if another effect with "when-gain" timing brings more Actions into play (such as Duplicate being called), the player can choose to resolve the effect bringing the Actions into play first, delaying the attempted VP gain until after those effects are resolved. This can lead to instances where when an Emporium is first gained, fewer than 5 Actions are in play, but the player still gets 2VP, since they have chosen to do so after bringing a sufficient amount of Actions into play. For example, with only four Actions in play, a player could buy an Emporium, choose to call Duplicate first, then gain 2VP (since they now have 5 Actions in play), then another 2VP from the Duplicated Emporium.