8 hours is the chosen number of hours they'd like to advertise to potential employees - that's basically the number of hours that would suit the store, and the vacancy that needs to be filled.
If you were actually awarded an 8 hour contract during after the interview and stuff, this is the minimum number of hours you would probably do a week - they may ring to ask you to work extra days where you're available, or even extend your shifts on certain days if it's a busy day. You can always say no, don't worry
In the interview, you can always tell them that you're always up for working more hours during the week - this will look good, as it shows interest in your job. If they wanted to then increase your contracted hours, they would.
If they don't, there's other options to increase your hours worked - you'll be able to access an online portal to see your contracted shifts for the forthcoming two weeks. It also tells you if there's any extra shifts available that you are allowed to work if you wanted to
And, there will always be people in your workplace trying to get people to cover their shift for many reasons. If you're after the hours, just spread the word and people will approach you about it
Hope that answers your question!