Let's Encrypt has published
a retrospective that covers the decade since it published its first
publicly trusted certificate in September 2015:

In March 2016, we issued our one millionth certificate. Just two years
later, in September 2018, we were issuing a million certificates every
day. In 2020 we reached a billion total certificates issued and as of
late 2025 we're frequently issuing ten million certificates per
day. We're now on track to reach a billion active sites, probably
sometime in the coming year.

https://lwn.net/Articles/1049965/
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