Esshup answers one line of thought I was having. I also wonder if they will live thru the winter if you have that thick of ice.
I would throw 10 in and see what happens. Monitor them, see if they either live thru the winter, or the summer. If they don't, then you might have to do a little more work to try and get them to live longer so you don't have to stock them multiple times per year.
If they do live and you find that it's to many, they are easy to catch and remove. If you still have leeches, then you know to add more the next time.
