Is there a way I can limit the lines a user can enter in a TextInput in kivy?
You can control whether the TextInput supports multiple lines with the multiline option. You can set multiline=True and then check the contents of the widget after an input event for a line count, optionally discarding a new line when necessary. You'll have to do the housekeeping yourself, but it's doable.
I don't see an easier way at this time to config the widget to not exceed a certain max number of lines. You'll have to limit the line count via your code.
Limiting lines in kivy TextInput?