The correct title for your question would be something like "how many different addresses are available in I2C when using 7-Bit address scheme".
If you review the I2C specification, you'll notice that some addresses are reserved, thus not all 7-Bit combination can be used.
How many slave devices can be connected is a quite different question. It depends primarly on the acceptable bus load. Usually the number is smaller 114. Often aspecific devices can only use a small set of preprogrammed addresses, so there are other limitations. If you can freely choose the device address, e.g. with µP slaves, you can also switch to 10-Bit addressing.