Number of nodes is not a problem. At least for security of the system. What could they do? Transmit invalid transaction / do not transmit a valid transaction.
Honest nodes and miners will ban those nodes straight away. They only good to spread the FUD across the system. And they already tried it with BitcoinXT. However, it is a psychological (social-engineering) attack not a technical one. What they tried to do, is give an impression that a lot of people moved to their client. And probably, most of those nodes were running on single server.
Could similar attack happen in the future? Yes, possible. Maybe, it's already underway with Bitcoin Classic. But those attacks not stand a chance against bitcoin protocol.
You're underestimating social-engineering attacks, which include political maneuvering. If you think governments won't kill a billion dollar industry through their own good intentions, I encourage you to check out Detroit sometime.
But it's not only social-engineering attacks. DDOS, physical attacks (even natural disasters), and viral attacks become much more effective when you're only dealing with a handful of nodes.