You could use a hashing system to produce a filename which contains all numbers that have that hash and choose a hashing algorithm that gives you your desired granularity. At it's simplest, you could just take the last digit of the number & store all numbers that end in (for example) 0 in "0.txt". Choose two (or more digits to further reduce the number of entries in each file.
Can you give a bit more detail about what your system as a whole is going to do?