@samy2 - there is an easier way than creating a custom column. Just set your excess tags in the Quality Check options to 1. Run the Excess tags check. Then change the search expression to this:
Code:
not marked:excess_tags tags:true
This is saying you dont want any books that have more than one tag, AND you want books tagged with at least one tag - i.e. you end up with books that have just one tag.