Creating an archive page with full year and month list
{% set groupedEntries = entriesInYear | group("postDate|date('F')") %}


Will give you an associative array where the keys are the months, and the values are the entries for that month.

Now you need to loop over all the months, not just the ones in the groupedEntries.

Make an array of all the months.

{% set allTheMonths = ['January', 'February', 'March', ... 'December'] %}


Make an array of all the months that have entries (twig's keys filter)

{% set monthsWithEntries = groupedEntries | keys %}


Loop over the months (all of them), and output either the number of entries (groupedEntries[month]|length) or 0

{% for month in allTheMonths %}
  {% if month in monthsWithEntries %}
    &lt;li&gt;&lt;a href="#"&gt;{{ month }}&amp;nbsp;({{ groupedEntries[month]|length }})&lt;/a&gt;&lt;/li&gt;
  {% else %}
    &lt;li&gt;&lt;a href="#"&gt;{{ month }}&amp;nbsp;(0)&lt;/a&gt;&lt;/li&gt;
  {% endif %}
{% endfor %}