The number of devices in use simultaneously depends upon your streaming plan
https://support.netflix.com/en/node/373
and, of course, of the speed of your internet connection.
Quote:
"We're constantly working with studios and distributors to get the rights to the movies and TV shows you want to see. There's no schedule - we add new titles as soon as we get them, so we have a New Arrivals page to let you know what's new on Netflix"