Today, there are roughly 1 Million second-level domains under `.io`, including `github.io` (i.e., all y'all's GitHub Pages), `kubernetes.io`, and a whole bunch of "clever" company names.
As with many ccTLDs, no reliable, public stats are available; some companies monitor DNS traffic to guesstimate the number. I (try to) track all TLD stats here:
https://www.netmeister.org/tldstats/
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