How can I use quantity selector on payment form??

The dropdown option won't let me make multiple purchases an option, 

You can add a quantity selector and set a fixed quantities or display a text box so your users enter the desired amount: 

Please check this guide that will help you with quantity selectors: https://www.jotform.com/help/225-How-to-Add-Options-to-Payment-Forms 

If you have questions, let us know.