There is a big difference between running a few sets on a copier and actually printing a job on a press. A large portion of the cost of printing a job is setup. If you order fewer copies than you need then order more a few days later, you will pay for two setups. The cost of setting up a typical printing job can range from a few dollars to several hundred dollars. There is also the scheduling factor.
Most printers keep a backlog of work that can be as short as a day or two to several weeks.
I know one large commercial printer that tries to keep enough work lined up to keep a six month backlog. Simply put, you will receive much more cost effective service if you order enough the first time. Many times, customers compose their projects using a desktop publishing program then are not happy when colors are not brighter and everything is not where it was on the screen. Ink on paper will not look the same as lighted pixels on a computer screen. Another thing that happens many times is that a customer sets up a project on his or her computer and the piece is composed for a certain desktop printer.