The answer to this question depends on the operating system of your forest.
- Windows 2000: 800 domains
- Windows 2003/2008: 1200 domains
These are Microsoft’s recommendations.
Another interesting note is that your domain controllers can only create 2,147,483,393 before they can no longer create any more new objects. I think it is safe to say that most organizations will not hit this limit in a domain controllers lifetime.