Remember that probably majority of those sites are either dead, not public and made when some people were learning how to make sites. You can put that number probably down to 10% active/public sites. If visitor number is 175m this might be unique visitors to the platform = one visitor might visit multiple sites.

Yes but in this case it would be much better to cite the number of "active sites" and not an almost meaningless 20M figure. Still, it's hard to define "active sites".

Then the number would be lower... Its all about marketing. "Unlimited", "hundreds of millions", "probably most of" sounds better than "5% of resources", "20 million active", "there is no data but we think a lot".

This

You're still assuming an even spread amongst the ones that are in that 1M. My point is it's usually hard to tell, and you should assume very few sites hold most visitors. Just keep re-applying the concept (of the 1m, 1000 account for 70% etc).