Both the 3dBrickStat and 3dROIstats commands can give you the number of voxels.
3dBrickStat -count -non-zero dset+orig
or
3dBrickStat -count -mask mask+orig dset+orig
Otherwise, 3dROIstats has an nzvoxels option for multiple ROIs to count the number of non-zero voxels for each mask value.
3dROIstats -nzvoxels -mask mask+orig dset+orig
You can see more about in the help for each command and in these threads:
[
afni.nimh.nih.gov]
[
afni.nimh.nih.gov]
Of course the number of voxels will vary with voxel size/resolution and subject for any anatomical ROI.
