You can authorize up to six devices on an account. The number of simultaneous sessions depends on your subscription level. Here are the details from Netflix FAQ:
Can I watch movies instantly on more than one PC or Netflix ready device?
Some membership plans allow you to watch simultaneously on more than one personal computer or Netflix ready device at the same time (for example, watching a movie on your laptop while another family member watches on a TV via a Netflix ready device in another room ).
1-disc plans may watch on one device at a time, 2-disc plans on up to two devices at the same time, 3-disc plans on up to three devices, and plans with four or more discs on up to four devices simultaneously.
In addition, due to requirements from content providers, there's a limit to the number of unique devices each account can use to watch instantly. Each Netflix account may watch instantly on up to six unique authorized devices, including personal computers and Netflix ready devices.