If you will skim through the postings on this message board you will find several from people who have gone on cruises, as in "truly single." Me, included, several times. Actually I love to cruise by myself.
As I always advise, seek out the maitre 'd' as soon as you board. He is always in a dining room taking care of table assignments for those with special requests. Tell him you want to be seated with other singles. His hours will be posted in the first day's schedule of events that you will find in your cabin...usually next to your life jacket. You should also see a time for the get-acquainted party for singles. If there is not one, then go find the Cruise Director and tell him/her you would like to help organize one.
As for predicting how many singles will be onboard, there's just no way. Sorry. Just make up your mind to have a great time, regardless. And be sure to post here when you return and tell us all about it.