Get List of all user agents

cat /var/log/access-log-file.log | awk -F'"' '{print $6}' | sort | uniq -c | sort -rn | head -n20000 > /home/output-file-path.txt

If log is in gz format
zcat /var/log/access-log-file.log | awk -F'"' '{print $6}' | sort | uniq -c | sort -rn | head -n20000 > /home/output-file-path.txt