All commands on the I2C bus are addressed to a specific I2C device (many have the option to set some pins high or low to select additional addresses and allow multiple devices to exist on the same bus). Each device must have a unique address so that only one device will respond at any one time. The PCF8591 starting address is 0x48, with additional addresses selectable by the three address pins to 0x4F. This allows up to eight PCF8591 devices to be used on the same bus.
If you decide to use the I2C_VC bus that is located on GPIO pins 27 and 2828 (or on the P5 header on Model A and Revision 2 Model B devices), you may need to add a 1k8 ohm pull-up resistor between the I2C lines and 3.3 V. These resistors are already present on the I2C bus on the GPIO connector. However, some I2C modules, including the PCF8591 module, have their own resistors fitted, so it will work without the extra resistors.