20 micro niche sites or 1 authority

Go for 20 Niche sites, in my experience im telling you
if you have one authority site, you have a lot of fear factors like click bomb and so on, which will put an end to your earnings, if you have more sites, you can remove ads from those sites alone, by this way you will have some revenue under your belt.

Be Wise and Play Safe.

what if you make 20 microniche sites, and after a while take your best MN site, add more content and transform it into authority site?

I agree with Ruger above, start with 20 MNSs or 5-10 and see which one takes off first. Then build on that one.