If I understand correctly, OpenType font files (
.otf) can possess a maximum of 65,535 glyphs.
What, if any, is the maximum number of glyphs an OpenType Collection file (
.otc) can hold?
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OpenType Collections are just bundles of multiple font files.
Each font within that collection is obviously limited by the maximum glyph limit, but as far as I'm aware there is no limit on the number of fonts contained within a collection. I can't find a defined limit either in any specification or in generators (i.e font editors).
Microsoft's OpenType specification defines the
numFonts header (which holds the number of contained fonts) as a
ULONG (a 32-bit unsigned integer) so technically there is a limit on the number. The maximun value of a 32-bit unsigned integer is 4,294,967,295. If we take that as the maximum number of fonts the maximum number of glyphs would be:
4,294,967,295 x 65,535 = 281,470,681,677,825
Your font collection would be unusable long before you reach that limit, though.
In reality you are more likely limited by the number of fonts your OS, or software can deal with and memory issues. Noto Sans CJK, for example comes in a number of different OTC collections. The "All-in-one super OTC" contains 36 font files and it is suggested "In limited memory case, use region specific OTF subsets." — which suggests memory is an issue long before you reach any maximum number of fonts.