View source for How to use predefined list of quantity instead of text input field on signup form

First of all you need to enable option 'allow user to change quantity' in product settings.
''aMember CP -> Products -> Manage Products -> (edit)''

You need edit your signup form at
''aMember CP -> Forms Editor -> (edit)''
and add Javascript brick to your form.

Then click ''configure'' link on this brick and put the following code in brick configuration:
<source>
jQuery(function(){
    jQuery('.am-product-qty').css({opacity: 0, width: 0, padding:0}).after('<select>\
        <option value="1">1</option>\
        <option value="2">2</option>\
        <option value="3">3</option>\
        <option value="4">4</option>\
        <option value="5">5</option>\
    </select>').next('select').change(function(){
        jQuery('.am-product-qty', jQuery(this).closest('label')).val(jQuery(this).val()).change();
    });
});
</source>

Save your form and test it.