In a previous article, that you can reach here, we covered how the Quantity selector works within Shopify.
If you have decided that your store would function optimally by calculating price depending on the number of days the customer has selected, and also capture the number of units then your theme will require customization.
This type of customization requires adding an additional field to capture the number of units selected. Although we have done this customization for many sites just 1 example will be shown below.
With this customization we have hidden the variant field and applied logic that will correctly select the variant based on the number of days chosen. The 'No. of days' field will display the amount of days that correspond to the date range chosen by the customer.
Additionally the customer can select a quantity as long as it doesn't surpass the current capacity level.
To check out more of this user's site you can click this link.
If you would like a quote on adding this functionality to your site please submit a ticket and let us know how you would like it setup.