This is one where you may need a custom resolver for GraphQL (or change the relationship over to a manyToOne), as we don’t currently have aggregations to support this currently.
Depending on the amount of tags you have you could run a really inefficient query to get a list of tags then run a simple count query (will need to add the count to schema as we don’t have it by default, see: Count with GraphQL - Strapi Developer Documentation) then you could construct the count using a tag filter.