You have to include an <#if value_has_next>,</#if>
statement.
This may help:
<#assign seq = ["winter", "spring", "summer", "autumn"]>
<#list seq as x>
${x_index + 1}. ${x}<#if x_has_next>,</#if>
</#list>
will output:
- winter,
- spring,
- summer,
- autumn
See this and more examples at: