Well if you're going to argue with words, there was one Holocaust but many holocausts:

> Extensive destruction of a group of animals or (especially) people; a large-scale massacre or slaughter.

However it's discussed if "Holocaust" is a good term to describe the Shoah:

> This use of the term has been criticised because it appears to imply that there was a voluntary religious purpose behind the Nazi actions

https://en.wiktionary.org/wiki/holocaust