How many months can I take unpaid maternity leave on active TN visa?
Currently my TN visa is active until 2019.
Can I stay in USA on unpaid maternity leave if my employer grants my leave and doesn't withdraw TN status? Please advice.
You can stay in US on TN as long as your company's policies allow and employee to be on leave for that specific reason (and keeps you employed as you say, which legally they only need to do for 12 weeks, by FMLA). It is like being on vacation. Such mat leave would not be considered being laid-off (or "benched") because there is no work for you.