How many people can use a Netflix account?

My sister got the password from her new boyfriend and someone else's, can I use it too?

Yes of course.

Netflix does not limit the number of registered devices, but users who are looking at the same time.

Depending on the subscription model, 2/3 or 4 people can watch at the same time.

It doesn't matter whether you look over the same profile (you can create 5 of them).

If you consider that some evenings, others watch Netflix at lunchtime, you can easily put 10 people on a 4-person subscription and you pretty much never see the message that too many are watching at the same time

OK