public_html/catalog/view/theme/journal3/template/journal3/checkout
Opencart 3
/home/deerghayuorganic/public_html/catalog/view/theme/journal3/template/journal3/checkout/register.twig
line 43
<div class="form-group required account-telephone">
<label class="control-label" for="input-telephone">{{ entry_telephone }}</label>
<input v-model="order_data.telephone" type="text" name="telephone" value="" placeholder="{{ entry_telephone }}" maxlength="10" id="input-telephone" class="form-control"/>
<span class="text-danger" v-if="error && error.telephone" v-html="error.telephone"></span>
</div>
ADD THIS
maxlength="10"