how many voxels there are in the head model in total, and how many of those voxels are involved in the solution
In this case- How to get this information?
There is no solution for doing this from the interface or the functions of Brainstorm.
You would need to read the data structures from the files on the hard drive (source maps, head models, cortex surface, MRI), and compute the metrics of interest by yourself.
For this, advanced skills in Matlab programming are required, as well as good familiarity with the Brainstorm workflows.
To get started with Brainstorm scripting:
https://neuroimage.usc.edu/brainstorm/Tutorials/Scripting