I have had as many as 250 in my network in the past when SEO benefits made it worth that approach, effort and expense.
Most of us have scaled back to under 20 due to managament issues and problems with engines like Google and Yahoo modifying their algorythems and penalizing cross linked networks or even owners of too many doamins in a particular niche. Even if you use proxy domains in an attempt to get around this both Google and Yahoo are top level registrars and have access to the registrant data as required by law. Proxy domains are designed to cut down on solictations not spoof the search engines.
My suggestion is to build a few high quality sites with unique content and valid and useful information for readers (not spider food) and update them on a regular basis. Links are still important but be very careful about their source, avoid three way exchanges and build inbounds at an organiclly sensible rate. Dont add 300 in one month etc…
I would also recommend you read all the articles by our resident expert Bob Rains. He even answers questions as time permits. He is one of the best in the business by a very wide margin and thats why we added him as a resource for CAP members.