This question is related to others measuring research progress, including: how many papers DeepMind will have published by mid-2019
By Jan 1, 2020, how many "milestone releases" will be listed on OpenAI's progress page?
This spreadsheet contains graphs, summary stats and a bibliography of past papers and milestone papers.
By year, we have the following milestone counts:
2019: 1 (as of April 4)
In the same period, they published 76 ordinary papers, and even more blog posts.