How can I ask for a quantity on my form and perform the calculations?

hello,

I want to be able to add a option that registrants can select and then select the number of them .... is that possible? 

So if they wanted 5 extra entrance tickets I can have an option which reads, "Additional Entrance Tickets" and then the ability to select the number of those they wish to purchase beside it. And the form auto-calculates this total.

Yes, that's possible to do, you may simply add a number field that will be shown either when the "Custom Selections" field is filled or when certain selection is made: 

This guide will help you showing/hiding the field: https://www.jotform.com/help/316-How-to-Show-or-Hide-Fields-Base-on-User-s-Answer 

Then you will only need to include the field to the calculation as shown below: 

Now, if you only need the "additional tickets" to be shown per selected option then you will need to add multiple number fields this way each option that is selected will show a different number field. 

Here's my cloned form where I applied this: https://form.jotform.com/80057754175965 

Feel free to clone it: https://www.jotform.com/help/42-How-to-Clone-an-Existing-Form-from-a-URL 

Also, I noticed that there are a lot of conditions on your form with the missing field error, please fix them or the other conditions may not work as expected. 

I hope this helps.