Right now, the only way to find out how many people are tagged with a specific tag in your nation, via the API, is to call the people method, which returns a paginated list of people with that tag. You then have to go through every page, incrementing the counter for every person. Needless to say this involves a fair few API calls for large tags.
There's already a tag index where you can see a list of tags - we just need that index to include a number too.
ConsideringOfficial response from Farz Sokhansanj
This is something we will consider but does not have a timeline at the moment.
However we are paying close attention to the most popular suggestions so please share and encourage others to follow and comment!