The raw count of websites, aka copies of code sitting on some server means nothing, and is bad metric. What matters, is either website view count, or some metric of developer time spend on developing those websites.
I think the percentage that the GP is referencing is from W3Techs [0]. It's currently 39.7% in fact, and that's only looking at the top 10 million websites [1], so it's not just a simple installation count across all sites ever created.
[0] https://w3techs.com/
[1] https://w3techs.com/technologies