Hey Lindsay, great question!
With the latest release (4.1) you can now set a global number of tickets for sale, regardless of the number of tickets are sold for each type of ticket. So, for example, if you set your global number of tickets to 12, you can then leave each ticket with an open-ended number of available tickets and, as each ticket is sold, it decreases from the pool of 12 tickets rather than a set number of tickets for each type.
We have a handy tutorial that covers global stock in much more detail–definitely worth checking out!
Does this help clarify things a bit? Please let me know. 🙂
Cheers!
Geoff