There are two types of sei, the number and the conjugation of essere.
How would you figure out which sei the person is talking about?
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It's not the only Italian word that has different meanings: we call them parole omografe, that is, with the same spelling. There are homographs also in English, like bow, but in Italian the situation is different, because same spelling implies “almost identical pronunciation”.
Some of the monosyllabic homographs are distinguished with an accent; in the past it used to be required to mark an accent also on the verbal forms dài and dànno, a rule which is nowadays not observed any more.
Words like venti, pesca or botte can be distinguished in speech by the fact that the different meanings are conveyed by the quality of the tonic vowel (but not all Italian speakers mark it and several even use the wrong vowel, open instead of closed or vice versa).
The numeral/verbal form sei is just one of the many homographs; in this case the quality of the vowel is the same, but the context will usually make clear the intended meaning.
Given that sei as a verb is singular and sei is a number so generally goes with plural nouns, it is next to impossible to mess it up.
Six crazy people
Simply looking at the words behind it, it'll be easy to tell. Also if the subject is expressed
Tu sei pazzo
that'll be even easier. If you have an example where you feel confused, post it so we can show you the reasoning behind it.