Confluent schema registry below version 6.1 does not provide any endpoint to count the total number of schema versions in the SR.

summarize_ccloud_sr_versions.py

# Get basic summary of versions in Confluent Cloud schema registry.
# You need to provide the SR url as well as SR apikey and api secret (lines 6 - 8 below).
import requests
from requests.auth import HTTPBasicAuth

sr_apikey = ''
sr_apisecret = ''
sr_url = ''

auth = HTTPBasicAuth(sr_apikey, sr_apisecret)
r = requests.get(sr_url + '/subjects', auth=auth)
subjects = r.json()
print("Found %d subjects" % len(subjects))
count = 0
print('Subject\tversions')
for subject in subjects:
  r = requests.get('%s/subjects/%s/versions' % (sr_url, subject), auth=auth)
  versions = r.json()
  print('%s\t\t%s' % (subject, versions))
  count += len(versions)
print('Total %d versions' % count)