You can address up to 128 chips theoretically. I2C peripheral chips always have pre-programmed addresses, you can't choose them yourself (some chips allow you to choose a few address bits yourself, but most bits are fixed). In practice this means that you can't connect many periphral chips of the same type to the bus (similar addresses!) at once.
If you use I2C for communication between controllers, you can set the addresses at will, and 128 devices are possible.
Personally, I think I2C is very useful (use it myself), but only if it stays inside the box (not for communicating to external devices) and if you need to control no more than about 20 peripheral chips.