The maximum number of zeroes a polynomial can have is equal to the degree of the polynomial.
The degree of the polynomial x6+4x3+5 is 6. Thus, it can have a maximum of 6 zeroes.
The degree of the polynomial x16+24x3 is 16 and hence, it can have a maximum of 16 zeroes.
The degree of the polynomial x99+1 is 99. Thus, it can have a maximum of 99 zeroes.
The degree of the polynomial x100−1 is 100. Hence, it can have a maximum of 100 zeroes.