Tracking Covid19 using gelocation

Hello everyone,
I’ve been using MongDB locally for 1 year now and it worked very fine for me as I only used in small local applicaitons, but this is my first experience to MongoAtlas
now I’m considering creating an app that will help tracking the virus using golcation, I already submit to the COVID-19 Battle Form and my project was approved.
I made few tests and I can tell that tha app is working now (10 users, for 1 weak), but I want to know what are the contrainst that I ll be facing as tha app usage will grow up.

the logic of the app is very simple it tracks its users using geolcation and if any of the users is declared covid+ the app send a warning message to all the users that may have encountered him in the past two weeks.
for my use case (10 users, 1 weak) it took over 3 minutes to get the result.
I’m using Node.js Mongoose on Heroku to connect to the database.
so can anybody help to get to know what are the best practices that I need to follow in order to make it working with few cost for a population of 23 Million users. (I am from Morocco)
Thanks in advance.