Below is the script for taking backup of archive log between sequence number 1000 to 1050

For standalone database:

RMAN> backup format '/archive/%d_%s_%p_%c_%t.arc.bkp' 
archivelog from sequence 1000 until sequence 1050;

For RAC database:

For RAC database, we need to mention the thread number( like 1 for node 1 and 2 for node 2)

RMAN> backup format '/archive/%d_%s_%p_%c_%t.arc.bkp' 
archivelog from sequence 1000 until sequence 1050 thread 2;