As of now (April 2022), there are four seasons of Full Metal Panic anime: the first season (adapts novels 1-3 and three side stories), Fumoffu (adapts several of the side stories and is entirely a comedy), The Second Raid (adapts the two Ending Day By Day novels, 4 & 5, and the OAV is a side story about Tessa), and Invisible Victory (adapts novels 7-9). Yes, they skipped Dancing Very Merry Christmas (6), but they DID make it into an audio drama!
... yes, this also means that the anime doesn't cover the entire story and ends on a cliffhanger. But there was more than a ten year gap between TSR and IV so who knows what the future holds?
24 episodes
Animated by Studio Gonzo, Adapts Fighting Boy Meets Girl, Running One Night Stand, Trembling Into The Blue, the sides stories ____, and has a three episode original arc, The Wind Blows Home.
Twelve episodes
Animated by Kyoto Animation (it's actually their first work), it first aired in Japan from August to November 2003 (although two half episodes were released as OVAs instead of aired on TV
Adapts multiple side stories and features an anime original episode, "A Goddess Comes to Japan - Hot Spring Chapter." Later, Gatoh wrote a version of it in one of the short story collections.
Thirteen episodes and one OVA
Animated by Kyoto Animation, adapts both volumes of Ending Day By Day with extra material to tie it into the first season for those who haven't read the novels. The OVA is based off a side story.
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Twelve episodes excluding recaps
The initials "IV" also make up the Roman numberal 4
Animated by Studio Xebec, adapts Continuining On My Own, Burning One Man Force, and Come/Gathering Make My Day; unfortunately this means it ends on a cliffhanger.