Not working
I'm asking v17

I have tried it in v17
this above is for only adding your class "clsrequired" for first 5 field

if you want to add required attribute for first 5 field then you have to try below code
<t t-if="i &lt;= 5">
<div class="form-group">
<input type="text" t-attf-id="field_#{i}"
class="form-control"
required="1"/>
</div>
</t>
<t t-else="">
<div class="form-group">
<input type="text" t-attf-id="field_#{i}"
class="form-control"/>
</div>
</t>

Thanks & regards

Thanks Yatrik