Aaaah, that makes sense. I think the flag of Scotland (󠁧󠁢󠁳󠁣󠁴󠁿) is the longest usable one I've seen, but you could stack near infinite items in Zalgo form on top of normal letters if you count those. I don't think Unicode has any restrictions on the amount of co combinatory characters, though most text parsers will probably enforce some kind of limit.