For backup reader threads, it's always the number of physical devices your database files are stored on (see http://www.sqlbackuprestore.com/backupr ... writes.htm
For SQL Backup writer threads, it depends. If you are backing up to a single file without the THREADCOUNT parameter, there's only 2 threads - 1 to read the backup data provided by SQL Server and compress/encrypt it, and another to write the processed data.
If you are backing up to multiple files, it's 2 x the number of files. If you are backing up to a single file using a THREADCOUNT value of 2 or more, it's the THREADCOUNT value + 1.