minimal sustainable human gene pool?

I've looked, but didn't find answers . . . .

Any ideas about the smallest possible human gene pool that is
sustainable?

I'm probably using the wrong terms, so somebody please correct me.

Here's what I mean:
Imagine that a very small group of humans is genetically isolated from
the rest of humanity for many generations. (European royalty, the
people on Pitcairn Island, and the Amish have provided real examples;
and science fiction writers have given us plenty more scenarios.)
What is the smallest possible size of the starting group which can
reasonably be expected to have genetically healthy descendants after
many generations?

Of course, we could start with one pair. After repeated inbreeding,
the offspring would likely survive and continue reproducing
indefinitely, but certain health limitations would soon become
apparent. How many individuals would we require in the starting
group, in order to make it possible for this never to happen? (Let's
assume that the future generations involved will agree to reproduce
according to whatever "safe genes" scheme we come up with.)

Ted Shoemaker

(No, this isn't a homework assignment.)

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