I think that Derek Halpern's article at DIYThemes, Why You Should Put All Your Eggs In One Basket, is pretty good advice for most beginning webmasters.
I am, overall, not a huge fan of diversification. Diversification makes sense for passive investors, but doesn't make nearly as much sense for active business owners.
Warren Buffet said "Diversification is a protection against ignorance. It makes very little sense for those who know what they are doing." He owns an average of 33 stocks at any one time -- and that is after a lifetime of investing. His top five stocks comprise approximately 73% of his portfolio. If Warren can only manage five serious stocks at one time... how many web sites can any of us manage and expect to do well with?
It seems to me that the right number for a beginner might be two. In the beginning, I enjoyed switching off between my tech site and my political site. And, of course, when Google beat the heck out of my political site, my tech site kept my spirits (and revenue) up.
Of course, you can own lots and lots of domains -- but you can't actively manage and improve them. I own a bit over 1,700 domains, but even with 19 full-time team members almost all of our domains are static. We have fewer than 50 web sites with more than 20 pages.
A decade from now, Google will (hopefully) be better at telling good web pages from crappy web pages. As that happens, the webmasters who focused their efforts on making fewer higher-quality websites will benefit financially.
Derek is wrong about one thing though -- you can't cheat the 10,000 hours. Learning the tricks and shortcuts is part of the 10,000 hours. It's already factored in.