You can display the number of all data files in the database with the SAPDBA function Tablespace administration ® Freespace and fragmentation of all tablespaces.
See also Freespace Statistics.

When you use the function Tablespace administration ® Alter tablespace Add Datafile ® Display current files, the number of data files per tablespace is displayed.
See Extending a Tablespace.

Also use the displays Tablespace administration ® Display all tablespaces and datafiles and Tablespace administration ® Display all character devices in /dsk to obtain an overview of the file distribution. These lists also show which files are on raw devices.

This information can also be displayed in the Performance Monitor. See online help for the Computing Center Management System.