Heroic figures in history are too numerous to enumerate.
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From Song-dynasty scholar Wang Mou's 野客丛书: 似此等语，不可枚举, 'expressions like these cannot be counted one by one.' The idiom kept 枚举 (to enumerate one by one, 枚 = measure word for small items) and evolved 不可 into the more literary 不胜 ('cannot bear to').
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Formal and written, natural in essays, news, speeches. Overwhelmingly used at sentence end as a concluding predicate after a list: 例子、好处、贡献、种类 + 不胜枚举. Spoken equivalent: 太多了，数不过来.
