Hi MAR-T; it would be hard to put a number to it but I wouldn't say there is a large percentage of singles--at any time of the year--unless it is specifically a "singles cruise."
But take heart: there are usually more singles on the shorter warm climate cruises. And Spring Break starts as early as mid-February at some schools. So I would think there would be a fair number of singles on your cruise.
Princess has plenty of activities for singles, and no doubt some extra ones for Valentine's week: so be sure to check your Princess Patter (activities schedule)--there will be mixers starting the first night.
Have a great cruise.