Formal nights vary between Cruise Lines and Itineraries. It is best to always check your pre-boarding documentation for any themed or formal nights on your sailing.
As a general guideline, there will be approximately:
1 Black Tie Evening during a 2-4 night cruise
2 Black Tie Evenings during at 5-7 night cruise
3 Black Tie Evenings during a 8-10 night cruise
4 Black Tie evenings during at 11-14 night cruise