The most common norm for live-action or animated series is a maximum of two or three opening and/or ending themes per season, and the ocassional special-event or Finale Credits tune. When a show has a large number of OP/ED songs in a relatively short period of time, we're witnessing a Credits Jukebox. Happens almost exclusively with Anime endings.

Can overlap with Evolving Credits. Not related to Credits Medley.

Anime

• If variations count, Akikan has a different arrangement of the ED every time, sometimes with guest singers. Last episode went to Miku Hatsune. In a case of Tropes Are Not Good, this might have been what made the studio skimp on the animation budget.
• Bokura ga Ita
• Ef a Tale of Memories
• Hanamaru Kindergarten
• Highschool of the Dead
• The Ichigo 100% OVAs.
• The TV version of Kyō no Go no Ni / Today In Class 5-2 has 5 closing themes for a Thirteen Episode Anime.
• Kyouran Kazoku Nikki has 8 ending themes for 26 episodes.
• Lucky Star would have the cast sing the opening or ending theme to some other show at the end of each episode, instead of having its own ED.
• Mawaru Penguindrum has 10 ending themes for 26 episodes. Every ending except the first lasts for one episode.
• Every episode of Mushishi has its own closing tune.
• Neon Genesis Evangelion apparently has more than 30 different versions of Fly Me To The Moon.
• Ore Imo has a different ending for each episode. It has twelve (plus four) episodes.
• School Days
• Heaven's Lost Property
• Togainu no Chi
• Nichijou only has one ending theme (albeit with three variations) for its first half, but the second half has a new ED theme every episode.
• The Idolmaster changes the ending theme every episode, with each theme being different songs from the games.

Live-Action TV

• Los Simuladores
• Weeds

Radio

• The CBC Radio 1 show Spark uses different Creative-Commons-licensed music for each episode.