Note: Mp3tag only takes care of individual files.
Only if a tagged file knows how many tracks are stored into it's folder, ...
then the filter can be used to detect files within folders, ...
where the total number of files is less than 2 resp. is equal to 1.
There is procedure, which can help to set trackcount per folder, ...
Tracknummern in Aktionen
Afterwards you can apply the Mp3tag filter ...
%TRACK% IS "1" AND %TOTALTRACKS% IS "1"
See also ...
Filter albums with less than 2 tracks
DD.20170217.1051.CET