Yep, there are 8 bits, but that doesn't mean that you can only control 8 relays. Let's take 2 bits as an example. With 2 bits you can control max. 4 relays because there are 4 possible binary combinations:
1. --> 00
2. --> 01
3. --> 10
4. --> 11
In order to control 4 relays in this example you would have to use an IC that converts binary input to decimal output. This IC would have 2 data inputs and, therefore, 4 outputs. Depending on the input combination, only one of the outputs would be enabled. Such ICs are 74HC155, 74HC237, HC4515, etc.
Of course, nothing forces you to use additional circuitry. I think the main reason why people usually use X bits for controlling X relays (or some other switches) is simply because it is cheaper and easier to do. I also do this.
I think I got a little bit off the topic here, right?
:arrow: My point is: no matter how many bits are available you *can* use up to 2^<No. of bits> possible combinations. How many will you use depends only on you and your needs.
Hope you understood some of this. If not, feel free to ask me and I will try to clarify things as much as I can.