Do you mediate with admob? They don’t have fill rate on their dashboard, and I haven’t used them much. Is it close to 90-95, or far from that?
I want to integrate them to 1-2 apps to see how they are doing with my apps 
mC is good choice. Fill rate for CPI ad networks works little different. The advertisers is paying for a certain number of installs which means that for a country like India, you may see the same ad if the advertiser has chosen to get installs from India. The result is that many a times you may see the same ad being repeated for country where less advertisers are present. (may be 20-30 times in the same app for the same user). I have seen it myself while testing my app.
Having said that, not all of my apps are earning revenue from mC, I see $0 earnings even after sending say 5000-10000 impressions. Same is the case with some apps using appnext or startapp.
Thanks for the info @javaexp, then I will send daily ~5k impressions and see how they perform in my case.
I’d say in the 80-90% region.