Webform with multiple of the same membership
In order to multiply/calculate Amounts based on webform fields, try webform_calculator module: https://www.drupal.org/project/webform_calculator

We've made some edits to support amounts being calculated over multiple pages - you can find what we're currently using for our projects here: https://github.com/KarinG/webform_calculator

How it works

The module adds a Formula webform component. So you can then configure a CiviCRM Contribution Amount or Membership Fee field to become a field of type Formula and specify the calculation with components.

Here's an example of a Processing Fee (which is a webform civicrm line item):


PS - I'm a bit confused though why you would want to buy multiple Memberships of the same type (in CiviCRM that would just mean multiple terms of one Membership Type) - but if you're looking to do Math the answer I've posted here using our latest webform_calculator module is the way to go.