There is a small performance cost associated with each additional user, so there is an upper limit to the number of users one instance can support. As far as I’m aware no project has hit this limit so we know we can support many thousands of users.
There are some downsides to sharing user accounts, including
- Analytics: you won’t be able to tell how individual users are performing
- Security: you won’t be able to audit who performed an action, and you won’t be able to block a specific person
- Performance: each device will replicate metadata created by the other people
- User experience: reports and messages will be marked as “read” incorrectly, and it will be impossible to configure the app differently for different people (eg: two people will be shown the same task).
I strongly recommend starting with one user account per person until you hit an issue. How many users are you anticipating?