how many distinct web sites are estimated to exist?

Sort of depends on how you define a "distinct Web site". A reasonable starting point is to look at the number of registered domains which currently stands at 62,669,892. Of course the number is artificially inflated since it includes plenty of domains that are inactive. Conversely, it is also artificially deflated as it doesn't include sub-domains, meaning that it counts sites like Yahoo! and Blogger as a single site even though they both include literally tens of thousands of sub-sites.

Perhaps what's most amazing about the Whois stats is the number of domains registered in the last 24 hours: 721,727 as of this writing!

240,000,000,000

did it in an assignment recently lol