Keyed-hash of a lot of small numbers
As long as the key is secret, your list of MACs is safe and the numbers are not guessable. 

You probably can truncate your MACs to 80 bits in order to save space, as collisions are highly unlikely for $2^{23}$ entries or so.

If your numbers do not repeat and fit into 128 bits, the simple Electronic Codebook (ECB) mode with AES should also do a good job, you just store the ciphertexts encrypted on the same key.