There were quite a few, the ones that immediately spring to mind are:
Side B of McDonald And Giles' album on Island had just one track, "Birdman", albeit comprised of 6 sections. Does this count?
Also the first side of This Is.. Gracious has just one track, "Supernova", although this has four sections. (The album was recorded in 1971 and scheduled for release but was delayed until the following year.)
There are a number of albums with just two tracks on a side, including King Crimson's "In The Court Of The Crimson King" and Wishbone Ash's debut LP.
"Moving Waves" by Focus has just one track, "Eruption" on side B.