Hi Trang,
How many digits are there? Well in binary, there are 65 digits, of course. :)
If you want to know how many digits there are in the baseten version of 2^{64} (I think that is what you meant) then look at this earlier answer we posted:
The question in that posting went the other direction (binary digits from decimal digits), but the principal is the same: Take the log in the target base (10 in your case) of the number (2^{64}), drop the decimal portion, then add 1.
In geekspeak: floor( log_{10}2^{64 } ) + 1 = 20 digits.
Hope this helps,
Stephen La Rocque.
