6

 A perfect number is a number that is equal to the sum of its proper factors.  The first perfect number is 6.  The proper factors of 6 are 1, 2, and 3.  1+2+3=6.  The next perfect number is 28.

Saint Augustine (354-430) wrote:- Six is a number perfect in itself, and not because G-d created all things in six days; rather, the converse is true. G-d created all things in six days because the number is perfect.

For more information on perfect numbers: Perfect_numbers