Generally you have a small number of bits to count the waiters, because the mutex state has to be a word you can CAS and so you have either 32 or 64 bits to pac

Generally you have a small number of bits to count the waiters, because the mutex state has to be a word you can CAS and so you have either 32 or 64 bits to pack all the state you need. If your counter saturates you lose track of the waiters, and you have to fallback somehow.