If you have the rman log file produced with a rman full backup and you want to know what the max sequence number in backup / archivelogs is for a rman duplicate database or recover until. This command assumes there are archivelog backups included in your rman log.
Here is a little unix command to help.
grep “sequence=” your-RMAN-Log-File-Name.log | awk -F ” ” ‘{print $5}’|cut -d= -f2|sort