I was trying to make my blog show "Authors" instead of "Author" when there are more than 1 authors for the same article.
The solution (thanks to @Avaris) is to set i = article.authors|count
and that can be used as follows
<div align="right">
{% set i = article.authors|count %}
{% if i != 1 %}
Authors:<br>
{% endif %}
{% if i == 1 %}
Author:<br>
{% endif %}
{% if article.authors %}
{% for author in article.authors %}
<a href="{{ SITEURL }}/{{ author.url }}">{{ author }}</a><br>
{% endfor %}
{% endif %}
</div>
This is different to Set variable in jinja because in Pelican I cannot find a command like lenght(authors)
to obtain a number to define a condition.