Yes, in theory, a company's whole float can be shorted. Short interest is the percentage of shares shorted compared to the float. Short interest can exceed 100%. For example, GameStop's peak short interest was 141.8% of its float on January 4.
The same shares can be borrowed and lent several times. For example, if an investor owns shares of stock XYZ, their broker can lend those shares to short sellers. These sellers then resell the shares, which could be purchased by another investor whose brokerage could, once again, lend them out to short sellers and which could be purchased by another investor whose brokerage could, once again, lend them out to short sellers, and this can go on forever until the whole float or more is shorted.
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