Add a quantity field to Packages & Services
Packages and services are things that would actually make sense to sell in different quantities, if an admin want to.
The infrastructure is already in place to support this feature, as submissions take in a quantity parameter. For now, such parameter is intentionally hard-coded to always be qual to 1.
Adding this is the easiest path to finally allow for an initial implementation of per seat and per site memberships, as additional users and sites can be sold as packages that increment the available count by one.
Challenges in implementing this are mostly focused on the UI. Both in the edit page for products and the fields on the registration or membership change forms.