The capacity is very dependant on environmental factors, and the theoritical limit has no value. For all you wanted to know about I2C and more, go to http://www-us.semiconductors.philips.com/i2c/suppo.../#appnotes. For your specific question select 9397 750 00954 The I2C-bus specification version 2.1 - January 2000. If you need for some reason to connect more than one device with the same I2C address Maxim has an appnote.
Have fun
Erik