I started a thread here awhile back which I've been updating alot.
https://gfy.com/showthread.php?t=1026884 You might find some good posts here.
I'd say focus on 15 for now and make them as best as possible. It's better to have a few that are super targeted in my opinion. If I could start all over I would focus on one niche to begin with. I'm in too far at this point though so I have to focus one a few. ;) good luck my friend and looking fwd to hearing your progress!