Adjusting ticket availability is a two-step process.
Under Tickets, Invitations, Registrations, select Edit next to the Ticket Type ("What are you giving out") > change quantity > Save & Next > close the window. Do not make any changes on the page that loads after Save & Next.
Then, select Edit next to the Ticket Stack ("How are you giving it out") > change quantity to distribute > Save & Next > Save & Close > Close.
The Ticket Type and Ticket Stack numbers should be updated. If not, refresh the page.