In reply to Blue Ridge:
With anchors, there is a lot of judgement involved and the old clichés do have their place.
You may hear the term 'SERENE' as an aim for all anchors. This stands for Strong, Equalized, Redundant, Efficient, (No) Extension.
With your question about using a single anchor point, the headings you want to look at are 'Redundant' and 'Efficient'. A single anchor point is not redundant and there is always the danger that something will go wrong (you forget to clip the rope, the sling gets cut on a sharp edge, the boulder rocks and releases the sling underneath it - as I believed has happened on Heaven Crack at Stanage). The other side of the coin is that if it's a massive tree or boulder which you're sure isn't going anywhere, are you wasting your time by backing it up? at a single-pitch crag it might not matter but in winter in the mountains, efficiency and not wasting time = safety. Backing up every bomber anchor may decrease the risk of anchor failure but over the course of a route may mean you are on the route for an hour longer. An hour longer of being exposed to the potential of bad weather, rockfall, other risks involved in climbing. Only your own judgement, which comes with time and experience, can decide what the right thing to do is.
Generally I always err on the side of caution but if I have 2 absolute bomber pieces, I'm probably not going to scrabble around to find a third. If it's a boulder or a thread and there is another easily-placed piece, I probably will back it up, if only to spread the load between 2 ropes on 2 points. If it's a tree and I have spare slings, I'll probably chuck another around it as it takes no time at all.
The other cliché is giving the parts of the anchor a mark that needs to add up to a total. One that makes the most sense to me is a mark out of 5 with a desired anchor total of 10. That means if you have 2 absolute sinkers then that's all you need. If there is any doubt about either of your 2 bits, you need at least 1 more. Typically, the exception with this system is that BFTs (big f'ing trees) or BFRs (Big f'ing rocks) get a mark of 10 outright.