Lpcm encoded data kept on an analog source
The theoretical max number of bits you can get is function of the bandwidth and the signal to noise ratio of your media. It's often referred to as "Channel Capacity", see
http://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theorem

In practice this depends a lot on the specific properties of your channel (transfer function, noise properties etc.) and there are many technologies you can deploy effectively deal with these properties.