If you just want to count the posts of a single tag, try this snippet:
<ul>
<!-- Get all posts-->
{{ $posts := where .Site.Pages "Type" "post" }}
<!-- (re)set the counter to 0 -->
{{ $.Scratch.Set "tagCounter" 0 }}
<!-- increment counter if post contains the current tag-->
{{ range $posts}}
{{ if in .Params.tags "TAGNAME" }}
{{ $.Scratch.Add "tagCounter" 1 }}
{{ end }}
{{ end }}
<li>{{ $.Scratch.Get "tagCounter" }} articles tagged with TAGNAME</li>
</ul>
Replace TAGNAME
with your single tag.