If you are digging a new hole you should aim to disturb as little ground as possible and make as small a hole as possible, going at least 600mm deep. One back will usually suffice in this instance.
I usually put in plenty of water and tip in half a bag, mix it with a batten or similar and then add any small hardcore you might have. Check the post is vertical in both planes with a spirit level and then add the remainder of the bag, again ensuring it is well mixed, adding a little more water if necessary.
If you are replacing posts, then I am assuming that the previous posts were concreted in?
If that is the case you are going to need 2 bags because the hole will now be bigger. Use as much of the original concrete as possible, breaking it into smallish pieces first. This recycles it and saves you having to get rid of it also.
Hope that helps.