Note: This may be off by a few thousand projects, but i hope to keep it accurate by occasionally manually changing it. Note 2: The project ID given in the URL is not actually how many projects were made before it. This is because all project ids from 3318396 to 10000000 were skipped when Scratch 2.0 released, which means the actual billionth project will be https://scratch.mit.edu/projects/1006681605/
