Whether the end result is 12 items or 1200 items, there's a lot of labor and supplies that go into a project before anything is printed.
The Full Screen Printing Process:
Artwork: Art is created/edited/prepared digitally at a high enough resolution to get a good print and carefully separated into the various colors needed to recreate the design in ink. Depending on the design this can take hours of specialized labor.
Film Output: Each color in a design is printed separately on special clear films called Positives. They're like negatives in traditional photography but positive and full size.
Screen Prep: The screens are very fine polyester mesh (up to 300 threads per inch). They have to be cleaned thoroughly, dried, coated with a special UV reactive emulsion, and dried again. (A speck of dust or other contaminate during this phase can ruin the final screen.)
Screen "Burning": Each screen is paired with a positive film and exposed to a strong UV light. The UV causes the emulsion coating to harden except where the image on the positive film blocks the light.
Washout: The burned screen is washed with water to rinse away the unhardened areas of emulsion, which creates a detailed stencil for ink to pass through. The screen then has to be dried again.
Inspection: The hardened emulsion is checked for problems. (Tiny holes, etc)
Almost There: Inks are prepared, Screen edges are taped to keep ink out of the corners, Screens get aligned for multi-color prints, and Test prints are made.
------- Now we're ready to print! -------
Printing: Each item is loaded on the press, ink is forced through the screen with a squeegee, and the ink is cured according to the ink system being used. For textile inks, curing usually means heating it to 320°F.
------- Done printing -------
Cleanup: Excess ink is scooped up. All ink and tape residue gets cleaned from the screens and tools.