An education is a wonderful thing. I'm getting an education. I hope to distill it into a couple of useful paragraphs.
Turbo molecular pumps (TMPs) are rated for gas flow at a certain chamber pressure. A high volume pump I am looking at is rated at 2,000 liters a second. How many ccs a second is that at STP (not Space-Time Productions - Standard Pressure and Temperature, which is 760 torr and 273.15°K) at a chamber pressure of 1E-7 torr?
2,000 l/s * 1,000 cc/l * 1E-7 torr * 1 atm/760 torr = 2.63E-4 cc/sec @ STP
Turbo pumps have a compression ratio rating. Let us look at what that means in terms of ultimate chamber pressure in terms of the worst case gas - molecular Hydrogen. The particular pump I am looking at is rated at an ultimate pressure (on the low side) of 1E-9 torr. If the pump has a compression ratio of 10,000 for hydrogen that means the outlet of the TMP must be held at a pressure of 1E-5 torr to reach a final pressure of 1E-9 torr.
We leave the design of an actual system as an exercise for the reader. Dig out them catalogs and get the slide rules slipin.