Count number selected in gift registry

I am using the gift registry to have groups schedule field trips. I would like for them to pay a $50 deposit per date that they select. How can I count the number of dates they select? 

Due to the way 'Gift Registry' Widget passes the selected options, I'm not sure if it can be used to achieve what you want.

You might try to use the Inventory Widget, since it provide an easier way to calculate the selected options. Quantity Gift Registry can also be used as another alternative.

I have cloned your form to pass the value of Gift Registry Widget to a calculation field - You can check from this link :https://form.jotform.com/61316683972968.