As of 2011, the typical hour-long drama has an open, plus six acts. This is up from just a few years ago, where the norm was four acts.
Half-hour multicamera comedies typically have an open, two acts, and a tag, while single-camera half-hour comedies can have either two or three acts sandwiched between the open and tag, depending on the show.
The “open” or “cold open” is the short segment you often see before the opening credits, which usually either sets up the plot directly or is completely unrelated to the upcoming episode — one extreme or the other. The “tag” is the short segment after an episode, which typically recaps what happened, wraps everything up, and/or has a joke relating to an earlier moment from the episode. Tags often play under credits.