On a normal day Lets Encrypt issues nearly two million certificates. In some of the worst scenarios, we might want to re-issue all of our certificates in a 24 hour period in order to avoid widespread disruptions. That means being prepared to issue 200 million certificates in one day, something no publicly trusted CA has ever done. In February of 2020 a bug affecting our compliance caused us to need to revoke and replace three million active certificates. We want to be prepared to respond as best we can to the most difficult situations that might arise.”]
Source: https://letsencrypt.org/2021/02/10/200m-certs-24hrs.html