I'm wokring on a mobile messenger app using Flutter. I'd write backend code in Node.js.
My app will have a list of users and each user can send request to other users to get paired with them. Each user can reject/accept other users' requests. If two users get paired, they can send message to each other or make a voice/video call. All messages transfered between users will be registered on the database for further analysis or detecting any criminal materials or misuses. I'd have about 2000 users, meaning the maximum online users are about 2000 people. The overal nature of the backend model is relational.
I need to choose a proper database for this app. My options are MongoDb and Sqlite. Which one is good for this project? Can Sqlite handle this project with that amount of users?
You might want to consult a lawyer, not a software-dev.
At least in the US and presuming a single company then users can have no expectation of privacy for company resources. Notifications to company employees are nothing more than a courtesy. Although perhaps also that they should keep the personal stuff off the company sites.
Assuming the database will be controlled by some hosted system that is only accessed by your backend system, then SQLite would be a resonable choice. See Implementation Limits For SQLite[^] for guidance on sizing.