line = '5320411,  996/7     W/O ABC ANO-20'

re.sub('^(S+)s+[0-9]{3}/[0-9]{1}', ',', line)

line = '5320411,  996/7 ,    W/O ABC ANO-20'

re.sub(r'(d+/d+)', r'1,', line)

# Output
'5320411,  996/7,     W/O ABC ANO-20'

(                   # Start of matching group 1
  d+               # Matches 1 or more digits
  /                # Matches /
  d+               # Matches 1 or more digits
)                   # End of matching group 1

import re
line = '5320411,  996/7     W/O ABC ANO-20'
new_line = re.sub('(?<=d{3}/d{1})s', ' ,', line)

'5320411,  996/7 ,    W/O ABC ANO-20'