That’s the page number. It’s an electronic only journal, so it has “article numbers” for each article rather than a page range. But you can cite it just like a page.

Technically, APA manual wants the word “Article” in front of these. That’s unique to that style and pretty dumb in my opinion—it really is just a page number. Even many APA journals ignore that rule.

@adamsmith does the "number:" feature work in other languages? For me it doesn't seem to do. It inserts "Article" even if I change the language.

@adamsmith thanks for the localization. Is it intentional that an abbreviation is used in German ("Art." instead of "Artikel")?

@adamsmith @bwiernik I would like to suggest using eid: e14903 instead of number: e14903 (or since some users might already use number: e14903 additionally add eid: e14903). This would minimize ambiguity (especially when exporting to LaTeX).

This is because there are articles with both, a number and an electronic identifier, e.g.,
- possibly all articles in all journals of the publisher MDPI, e.g., https://doi.org/10.3390/vaccines11071277
- possibly all articles in all journals of the publisher IOP Science, e.g., https://doi.org/10.1088/2040-8986/acdbdd
- ...

Why to choose "eid"?
It seams quite suitable and the biblatex Package defines the field eid as:
"The electronic identifier of an @article or chapter-like section of a larger work often called ‘article number’, ‘paper number’ or the like. This field may replace the pages field for journals deviating from the classic pagination scheme of printed journals by only enumerating articles or papers and not pages.
Not to be confused with number, which for @articles subdivides the volume." (https://ctan.org/pkg/biblatex, p. 20)

Thank you for your answer!

I guess the discussion you were referring to is this one: https://forums.zotero.org/discussion/comment/437885/#Comment_437885
for anybody who wants to read it.