I was unable to find a Cat Stevens cartoon about the Numbers album, however, I was able to find a cartoon that was released in 1977 featuring the 1971 song Moonshadow. You can find it on Youtube here.
Reading into the Numbers album, it seems as though it was a critical and commercial failure (at least compared to Stevens' previous records), so I'd find it unlikely that his record labels would allow/fund the creation of a cartoon for such an album. This makes me think that you might be confusing the release of Numbers in 1975 with the cartoon from 1977, since they came out around the same time.
All of this having been said, I could be quite wrong, as this is all I was able to find online, and I wasn't alive when any of these things were first released. But even if this isn't the correct answer, hopefully it points you in the right direction!