ARROW-18220: [Dev] Remove a magic number for the default parallel level in downloader (#14563)
ARROW-18220: [Dev] Remove a magic number for the default parallel level in downloader (#14563)

Authored-by: Sutou Kouhei <73218711cdd5d5c30ac66da3cc4d9d9204caf34b@clear-code.com>
Signed-off-by: Sutou Kouhei <73218711cdd5d5c30ac66da3cc4d9d9204caf34b@clear-code.com>
kou/arrow,kou/arrow,apache/arrow,apache/arrow,kou/arrow,apache/arrow,kou/arrow,apache/arrow,apache/arrow,apache/arrow,kou/arrow,apache/arrow,apache/arrow,kou/arrow,apache/arrow,kou/arrow,kou/arrow,kou/arrow,kou/arrow,apache/arrow,kou/arrow,apache/arrow,apache/arrow,kou/arrow,apache/arrow,kou/arrow,apache/arrow,kou/arrow