There are quite a number of important metrics that you need to be tracking and improving upon in order to make your app a true success. Always be looking at the wider picture, and evaluating how each metric has an effect on one another.

If your team is small, or you are an indie developer working alone, then I’d recommend starting by iterating your product with the focus on increasing engagement, retention, and your average user LTV. You don’t need to have millions of users on board in order to build a truly great mobile app. Test heavily, and make data driven decisions in order to position yourself in a place where you can start to invest in acquisition with the confidence in your product’s quality and monetisation.