When I last took a ten-night cruise (aboard MV Sun Princess over Halloween last year), the evening dress actually deviated from what's printed in the Cruise AnswerBook. The chart in the Cruise AnswerBook shows two formal nights and three semiformal nights for a ten-night cruise, but we had the reverse.
I have not heard of any elimination of semiformal nights on cruises longer than seven days. Thus, I would expect at two or three semiformal evenings during your cruise.
BTW, it's always good to bring an extra semiformal outfit (which really means just an extra shirt and an extra necktie for gentlemen). On several cruises, I have had opportunities to participate in special events that have required semiformal attire. Additionally, dinner at the captain's table -- if you are fortunate enough to receive an invitation (which social etiquette says that no passenger may ever refuse) -- always requires semiformal attire on an evening that's otherwise casual.