Setting your ticket quantity is a key portion of event creation. Always start by creating a New Ticket Type. One of the first distinctions you will have to make is whether that ticket is going to be "Reserved" or "General Admission."
If you choose General Admission, a quantity box will appear where you can definite the quantity for that ticket type. It is important to note that your price levels within that ticket type will all pull from that defined quantity.
If instead, you select Reserved, the quantity will be tied to the number of seats you select. This is done in the seating tab. After you pull in a seating chart, select ticket types and simply click & drag over a selection of seats. This creates the quantity.