You can daisy-chain multiple pumps. IIRC, people tend to do this most with the Laing DDC (Swiftech MCP335) style pumps.
One should be sufficient for a single CPU and graphics card. For two graphics cards, one extra pump would probably be a good idea. For something like CPU, northbridge, MOSFETs, two or three graphics cards, and other miscellaneous stuff, you're going to want to run multiple loops (probably three), each with at least one pump.