If you mean the active websites there is no real valid number for this (parked pages, deactivated accounts, redirects....) If you mean just domains is easier
I typed the letter a in google and got 18,340,000,000 pages, but one site can have many pages. I don't know how to find out how many sites there are on the web at the moment. The number is changing as we speak (write).
As i said before you can't really count them with any known method Even if you do count the big TLDs like .com.net.... you still miss all the ccTLDs websites
I am sure this is an ever increasing number from second to second. Quite hard to give you a solid number.