I2S TDM channels maximum bandwidth
I2S as such is limited to two channels, but there are various extensions.

For example there are variations on the theme of TDM, often used with DSP parts and usually good for 16 channels or so on an interface that electrically looks like I2S, support for these formats is common on DSP chips and the better sorts of ADC and DAC parts, but somewhat rare in the microprocessor world. 

It is of course possible to share LRClk and BClk and just run multiple parallel data lines, and this is very common.

Usually these links are within a board or at most within a chassis, the clue being in the 'Inter IC Sound' name, for larger scale links, something self clocking is usually better, MADI/AES67/Custom SERDES, there are a whole world of choices.