If you will have even two concurrent users, you must account for the possibility that they are both interacting with the spreadsheet at the same time. The best way to handle this is to use the Lock method to prevent concurrent edits. See [this Medium article](https://medium.com/google-developer-experts/tips-on-building-a-reliable-secure-scalable-architecture-using-google-apps-script-615afd4d4066) for more issues with scalability.