[Django]-Max number of clients reached celery

You are probably hitting a plan limit on the Redis server itself. If you are using the Heroku Redis addon, consult the plan descriptions: https://elements.heroku.com/addons/heroku-redis

You should expect a minimum of 1 connection for each dyno – for web dynos to queue stuff up, and the celery workers to pop stuff off. 20 web + 5 celery easily puts you over the for the free tier.