The answer is simple – just one. Beware we speak of WRITE access as well here – READ access is available to as many users as you want. But the moment the first user logs in into Gaia he automatically gets the write/read access and all the later logins (it actually can be the same username logging in) get just READ only mode. Any user (if has admin priveleges) can acquire or override the database lock from the current user and this way have write/read role. Of course all the unsaved changes made by the previous user will be lost.