I assume you are using MySQL Workbench? I do not have an example of accessing DocuWare with that, since our database is MS SQL Server. But the methodology is the same.
Assuming you are connected, you need to go to the correct database, or "schema" as MySQL calls them. I have attached a picture of accessing a MySQL database I have set up to store weather data from my personal weather station at home, just to give a feel for how the databases are listed.
I have also attached what a DocuWare database listing looks like in SQL Server. AS you can see, file cabinet data is in a database/scheme called "dwdata". Expand that database and find the table representing the file cabinet. It should have a similar name, though certain characters are converted to underscores and the name will be in all capital letters.
Once you have the table visible, right-click on it and choose "Select Rows" from the menu. This will generate a query to return all rows in the table, such as:
SELECT * FROM <schema name>.<table name>;
If you just want the count of records, you can use this for your query:
SELECT COUNT(*) AS NumRecs FROM <schema name>.<table name>;
That will just return the singleton field denoting th enumber of records in the table, and that should be your number of documents in the file cabinet.
Hope this helps!