I'm going the opposite direction from you. After six solo cruises I'm taking my kids (DD 14 and DS 16), plus their best friends, on Voyager next month and am wondering how to keep them busy. How did you find the kid entertainment on your past cruises.
Now, to answer your question, the number of solo cruisers varies widely cruise by cruise. On my first cruise there were 1600 passengers and according to the HD I was the only solo cruiser (cabin to myself) but on my second cruise I met 15 and heard there were others. Be sure and check in on your roll call.
I don't seek out other solo/single cruisers in particular but agree the piano bar attracts a lot of solo/single or just those on their own for a couple of hours. Generally speaking, I do what interests me and meet people along the way. People are so friendly on cruises that it matters little whether they are solo/single or not. In AK I spent several nights in the casino with the husbands of two women I met. The wives didn't want to gamble and I did. I've also gone on an excursion with the wife of a couple while the hubby was playing golf. I've hung out with couples, families and other solos. It was all great fun.
You'll have a great time. The only problem with solo travel is it is addictive.