Thank you for your answers.
I use the following solution:
<h1>{{ .Title }}</h1>
<p>{{ printf "%d articles" (len .Pages)}}</p>
<hr>
{{ range .Pages }}<p><a href="{{ .Permalink }}">{{ .Title }}</a><br>{{ if .Params.date }}<span class="date">{{.Date.Format "02 Jan 2006"}}</span>{{ end }}{{ if .Description }}<br>{{ .Description }}{{ end }}</p><hr>
{{ end -}}
I found this solution here: https://discourse.gohugo.io/t/count-number-of-pages-matching-a-term-in-a-page-of-kind-term-taxonomy-html/35376/2