Function addressable_glyphs 

pub fn addressable_glyphs(font: &BitmapFont) -> u32

The number of atlas slots font occupies: its glyph count, capped at the 256 a u8 glyph index can address (see BitmapFont::rows).

A font that declares more glyphs than that has no way to name them, so the atlas doesn’t reserve slots for them either.