On a typical 14 night cruise, there are usually 4 formal, 4 informal and 6 casual nights. Further details in P&O brochur.
Depending on how long duration of cruise, several casual, several smart, and 2 or 3 formal. I do hope new cruisers comply with the formal or go informal dining as it does annoy me when I have made the effort and others do not. Little bit like a fancy dress party - when you have not made the effort you feel the fool! Seriously enjoy but do not think you can just go in to dinner with your shorts and tea shirt!!
As people have said there is usually a mixture of formal, informal and smart casual. The only exception to this is in the Caribbean (fly cruises) where in a 14 day cruise there will be 4 formal and the rest smart casual.