database design for storing individual users' items
A general rule of thumb is that you will have a table for each type of thing that you are recording information about.  Each instance of a thing will be a record in a table.

It would probably* be a terrible idea to have one table per user.  Have one table with USERS and one table with THINGS.  In the THINGS table you will have a foreign key which is a column containing a user_id that points at the USERS record for the owner of the thing.

Relational databases are designed specifically to make this kind of structure easy and fast to work with.  You shouldn't* ever need to worry about how many users there are or how many things they have.

* NOTE: Your requirements are very sparse.  These are rules of thumb.