My wife and I use the App I'm making with our cell phones. I installed the app on my wife's phone using the .apk file. Both phones access the same tags on the Cloud and that's fine. If, however, the app is used by a user who needs to access their own tags, should the names of the tags be changed (coupling it to a userid) or should another technique be used? Moreover if hundred of users will use the App (dreams) each with own tags, is that a problem?
Probably could become a problem if you use the default server using the CloudDB. Hundreds of users will probably cause issues with overloading the server unless you use a Redis database instead of the default server (intended for small Projects and non commercial usage).
This is up to you. Both ways could probably be coded to work. 
What will work best partially depends on whether each Tag represents a List; what the app does etc.
An example using a limited number of users Social Distancing ... a CloudDB / Location Marker Tutorial