Calculate length of custom dataset extras instead using harcoded value

ckanext/odsh/templates/organization/snippets/organization_form.html

@@ -47,8 +47,9 @@
     {{ form.input_address('gnd','GND URI', value=extras.gnd, index=6, placeholder='https://d-nb.info/gnd/1136109587', type='text', attrs={}) }}
 
     {% for extra in data.extras %}
-      {% if extra.key not in ['person','street','location','telephone','mail','web','gnd'] %}
-        {% set prefix = 'extras__%d__' % (loop.index0 + 7) %}
+      {% set custom_extras = ['person','street','location','telephone','mail','web','gnd'] %}
+      {% if extra.key not in custom_extras %}
+        {% set prefix = 'extras__%d__' % (loop.index0 + (custom_extras|count)) %}
         {{ form.custom(
           names=(prefix ~ 'key', prefix ~ 'value', prefix ~ 'deleted'),
           id='field-extras-%d' % loop.index,
@@ -65,7 +66,7 @@
     {% if empty_extras <= 0 %}{% set empty_extras = 1 %}{% endif %}
   
     {% for extra in range(total_extras, total_extras + empty_extras) %}
-      {% set index = loop.index0 + (data.extras|count) + 7 %}
+      {% set index = loop.index0 + (data.extras|count) + (custom_extras|count) %}
       {% set prefix = 'extras__%d__' % index %}
       {{ form.custom(
         names=(prefix ~ 'key', prefix ~ 'value', prefix ~ 'deleted'),