
To insert a comma or other separator between Tags or Categories in the Template (%post_tag% or %category%) use the :after
CSS pseudo class and add this selector to your Customizer.
CSS to add a comma between Tags:
.cat-post-tax-post_tag a:not(:last-child):after
{
content:",";
}
CSS to add a comma between Categories:
.cat-post-tax-category a:not(:last-child):after
{
content:",";
}