Here's the answer:
The University of Ottawa published research tracking the number of research papers published going back to 1665.
"According to research from the University of Ottawa, in 2009 we
passed the 50 million mark in terms of the total number of science
papers published since 1665, and approximately 2.5 million new
scientific papers are published each year."
Additionally, Derek De Solla Price in his book Little Science, Big Science and Beyond (where he proposed the concept of a science to study science) presented a graph of the number journals going back to 1665. (Given the year equivalence - it appears the University of Ottawa was using his data.)
Thus...
Combining these two sources together, we can estimate the average number of articles per journal (i.e. De Solla Price estimates there would be ~1M journals in 2000 if the rate from the 1700s continued - and Ottawa estimates there are ~50M articles in 2009...so we might expect there is somewhere between 25-50 articles on average per journal.)