Yeah others can chime in here, but I do not know a way you would accomplish this easily. You really want to do some kind of weighted ranking of items which is not something that is very straight forward to do. Below I shared a couple of alternative directions you could possibly look down.
First, you could write a function in SQL to grab the count of relevant keywords and return as a value in the query so you can sort on that. (probably easier to use a stored procedure to do the query and call the function). The big caveat on this is doing too much at the Kentico database level, you would just want to document well so you don't lose changes during subsequent version upgrades.
The second option would be to do this through code and count the relevant tags from there. You could look at the code behind the Tag Cloud webpart and back track from there as the tags show at different sizes based on how many items are 'tagged' with it. This could at least give you a code direction.
Wish I could of given you an easy answer but you have a tough one there.