You can see how many contacts have a certain Tag on the side of the tag in the Tags menu. Find the tag you are looking for by searching for the same in the search tags, search form.
Also, you can click to open the tag and go to the Tag Log tab. You will be able to see all contacts that have been added or removed from the tag as well as the number of contacts that have this tag. If your user account has permission, you have the option to Export a CSV version of the contacts in this tag.