ADC with I2C interface with multiple addresses
You didn't specify the A/D speed or accuracy, so any A/D will do.

One possibility is to use A/D chips with multiple input channels.  You can find them with 8 or 16 or even more inputs.  These are essentially a A/D with a analog mux front end integrated into a single package.  Four 8x A/Ds would do it, and you can probably find some that do IIC and have at least 2 address bits that can be set via the pins.

Another possibility is to use a small micro with a large number of A/D channels.  This is like the previous case in that there is a analog mux integrated with the A/D, but there is also a microcontroller integrated with it.  The integrated micro can then do the continuous sampling, perhaps some low pass filtering, and communication back to the master.  Now you're not limited to IIC, although many micros have IIC hardware that can act as a slave device.  Since the address is set by firmware, you can have as many of these on a IIC bus as it can electrically handle.  However, no more than 2 would be needed since a number of micros have 16 or more A/D channels.