WordNet

https://wordnet.princeton.edu/download/current-version#nix

From what I can see there is currently no thesaurus graphical or command line, this one is command based and can be used by other tools like emacs.

Seems to be open but has its own license saying it can be used unencumbered.
https://wordnet.princeton.edu/license-and-commercial-use

No idea how many but as its a tool that is often used indirectly by other software its more about enabling useful functionality.

The tarball is at this location http://wordnetcode.princeton.edu/3.0/WordNet-3.0.tar.gz