At least three if not four or five. If LA County (the most populous in CA and the US) were to secede and become its own state it would be the 9th largest in the nation (based on 2005 Census estimates). As its own state it would probably elect 15 or 16 congressman and have 17 or 18 electoral votes. What happens to the new LA-less California then? Not only would it still be in the top ten most populous states, it would also remain at the number one spot with about 4 million more people than Texas (the next largest state).
I would actually argue for splitting New York, Texas, and Florida as well. These states weild too much power in the House and Electoral College. The fact that these states are some of the worst gerrymandered in the country makes me even more uneasy. The governments of only 4 states should not be in control of drawing districts for nearly a third of our congressmen.