The number of new self-published titles that have both ISBNs and BISAC codes was 2,298,004 in 2021, a decline from the two previous years, but still well above the 1,551,391 titles registered with Bowker in 2018. In 2021, there were 2,300,336 new self-published titles with only ISBNs, a 15% decline from 2020. The number of self-published ISBNs issued topped one million for the first time in 2017 and had a huge spike in 2019, which Bowker attributed to a large order from a major self-publishing platform.
Source: Self-Publishing Is Thriving, According to Bowker Report