You can watch Netflix even without an unlimited data plan but you'll be able to only watch the amount of Netflix videos and movies that your limited data allows.
So for example if you have a 100 GB data plan then you'll be able to stream around 99 to 100 movies on Netflix before you run out of data.
That is of course depending on the quality of the movie as well because higher quality movies on Netflix use more data.
But usually 1 Netflix movie will use around 1 GB of data.
Having an unlimited data plan is best though when streaming Netflix or Hulu or other movie streaming content but you don't have to have unlimited data.