The blockchain launched in March 2016, and HF16 was in December 2016, so literally there were 16 forks in the first year (9 months actually). Since 2017 (3 years) there have only been 6 hard forks, an average of two per year. A couple of those did nothing but fix emergency bugs so in reality the number of planned hard forks that can be used to improve the blockchain has been less than two per year.