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Official ISO programming standards that describe programming language syntax etc often use <sub>_opt_</sub> to mark an argument as optional (with "OPT" in italics). So I would propose to write

> ARRAY[data-type, count<sub>_opt_</sub>]

This has the advantage that the abbreviation "OPT" is kind of self-explanatory and you don't have to write an explicit note about count being optional.

The disadvantage is that you need some manner of formatted text or HTML (`<sub>`) to support this, so it won't work in raw source code comments/raw Doxygen etc.

I wouldn't use italics for the optional parameter itself (_count_), because that's very ambiguous. Italics can be used for a lot of diverse things: emphasis, place-holders, formal definition of technical terms, source code identifiers and so on.