Depends how you want to use it. You can have up to four devices on your account to watch NOW TV on, and you can watch NOW TV on up to two at the same time. The stick is designed to be easy to unplug and move around so you could just buy one and move it from TV to TV as and when. Or you could buy several and leave them permanently plugged into each TV.
I think you're right it doesn't give you the option to choose how many you want when you purchase a stick on the website, but does it let you go back and purchase another one afterwards? I would expect it to. Alternately they are sold in several places such as Argos.