According to the AniDB, episode 22 is the last episode, followed a week later by a 'summer special', or the 23rd ep. The final episode, ep 24, shall be released as an OVA on the final disk release of Clannad in July. Yes, July.
So, all of you Clannad watchers will now have to add the Clannad OVA
to your plan to watch lists. (Unless.. you don't want to? Not forcing you)
Link to Wiki for extra confirmation!