There is are 13 Unicode astral planes (though only two of them have characters assigned so far), and it they do indeed encode poorly in some environments: the planes other than plane 0, the basic multilingual plane, are informally known as "the astral planes".

There's 16 astral planes (U+1xxxx to U+10xxxx), of which 3 have characters assigned...

* plane 1 is the supplementary multilingual plane

* plane 2 is the supplementary ideographic plane

* plane E is the supplement­ary special-purpose plane

* planes F and 10 are private-use planes

Perhaps you wrote from memory.

Thanks for providing the correct details. I was indeed writing from memory.