You can place ads in an app. Just make sure it’s not an earning app or a webviewer, and that it complies with the Play Store’s other requirements.
Advertisers pay to have their ads displayed to as many people as possible. Therefore apps that have few daily active users probably won’t show any ads. The ad can be requested by the app but Admob will not display any.
However, once an app has many daily active users (at least 100, preferably 1000+) then ad requests start to be fulfilled more frequently.
Also the country your users are based in can affect the number of ads displayed. People from Western countries are far more likely to be targeted as most advertisements will be for Western products or services that can only be afforded by Westerners. (Please note I am not trying to be offensive towards poorer countries or people, but it’s just a fact that the West is richer than developing countries.)
So when you create an app it’s best not to have any ads and instead focus on quality and usefulness to attract as many users as possible. Once you start getting good reviews and there are at least 1000 daily active users you can start displaying ads. You will lose some users but if you have a loyal base and your app is genuinely awesome then you will continue to attract more users who will see more ads and you will thus earn more.
Note: number of installs means how many times the app has been installed. Daily active users are the people who use it on a daily basis. Not everyone who has installed an app uses it every day.