Adapted numbering level according to quarto's two level cross-referencing mechanism from quarto 1.4 onwards that adds a crossref key to yaml.
For books, custom blocks are numbered by chapter by default. For single documents or websites, the numbering is consecutive. This can be changed with the crossref yaml key:
crossref:
chapters: true
turns on numbering by section (level 1 header) for other documents than books, and setting crossref.chapter to false in books switches numbering by chapters off.
Number prefix can be overridden by setting a yaml key numberprefix to the desired string value. This also works as div-block attribute.