You can allocate up to 255 channels; each channel can read up to 64 files in parallel.
You can control the degree of parallelism within a job by the number of channels that you allocate.
Allocating multiple channels simultaneously allows a single job to read or write multiple backup sets or disk copies in parallel, which each channel operating on a separate backup set or copy.
When making backups to disk, the guideline is to allocate one channel for each output device
If RMAN is writing to a striped file system or an ASM disk group, however, then multiple channels can improve performance.
When backing up to tape, the guideline is that the number of tape channels should equal the number of tape devices divided by the number of duplexed copies
Using Multiple Channels in Oracle
automatic channel allocation: The ability of RMAN to perform backup and restore tasks without requiring the use of the ALLOCATE CHANNNEL command.
You can use the CONFIGURE command to specify disk and tape channels.
Then, you can issue commands such as BACKUP and RESTORE at the RMAN command prompt without manually allocating channels. RMAN uses
whatever configured channels that it needs to execute the commands.