How many users can log in via my app?

Thanks for the feedback,

However, I am feeling a little dizzy after 3 weeks of development and not be sure that people would be able to log in through my app… Could you please confirm how possible is to get the increase of the users and process, in order to take a decision if it´s worthy keep through Strava? 

Thanks in advance

The page mentioned above is describing all you need to do. Make sure that the app complies with the API terms and brand guidelines, and then fill submit the Developer Program form that's also linked on that page. It's the only way to get the athlete limit increased (and hence let users log in), so it's up to you.