I have used Netflix on my pc, my ps3, my iPod touch, a laptop, and 2 different iPads that are in my house. I have never had an issue with using a new device. I have never tried using it on multiple devices at the same time though.
The maximum number of devices you can connect with Netflix on one account is four. This is usually on the 4-or-greater-discs-out-at-a-time plan. For those with 2-discs-out-at-a-time plan, they can only connect two.