Q:

ANTONYMS to "fixed"

A dict gives lots of words as antonyms to "fixed"

agitated, flexible, indefinite, insecure, loose, movable, moving, soft, undecided, unfixed, unstable, broken, changeable, impermanent, in disrepair, irresolute

I am trying to distinguish some of them:

The number of verb tenses is fixed, which is 12. In contrast, the number of active users on ELL is not fixed.

In this context, which words could be used?

A:

The most obvious choice there is "variable".
"The number of active users is variable."
 The only one of the other suggested words that fits is "changeable".
Note that many of the words have possible negative (e.g.,"unstable") and positive ("flexible") connotations.

