I wasn't talking about diversity, rather about how much people are needed to sustain and industrial civilization. Perhaps this number is much greater than number of people you need for diversity (several hundred people populated the whole pre-Columbian americas and they didn't have terrible problems with diversity)
At the moment? Large enough that it realistically can't be done. Millions and millions. Millions of tons of equipment, too.
Emerging technologies (nanotech...) may be able to reduce this, but for the moment it's perfectly realistic to consider scenarios where new colonies usually regress to 18th/19th century technology for a few generations. This, of course, makes colonizing uninhabitable planets such as Mars... hard.
Mars we could probably do, it'd just be costly. An interstellar variant? Not likely.
