If you are using Twos Complement representation, the largest positive number you can contain in N bits is: (2**(N-1)-1

In this case (2**999)-1

For unsigned integers it is (2**N)-1 or in this case (2**1000)-1

The approximate number of digits is log base 10 of 2 times 1000

of (0.30102999566) * 1000 or about 301 digits.

It depends on which bits it has. What is the number in binary? I could figure it out.

1000.

Do you perhaps mean "digits"?