I am a nappy advisor. I always recommend 20. In teh "olden days" our mums and grandmas would have bought 2 dozen terry squares, but 20 is fine. 20 works on a 3 day washing cycle - once the initial weeks are over and is a full nappy load. You can get away with maybe buying 15, especially if your baby is a bit older, and wash every 2 days but you may need to find a few extra bits and pieces to make up a full washing load.
For most of us here it isn't so much need as want! Don't buy a pack of anything - have a look around at what is available and try a few of what you fancy. When you know what works well for you, buy a few more.
If you are on guernsey - denise will be a great help to you - she will be able to show you all the different nappies and figure out what suits you best.
BG are a good nappy - very popular but there are lots more on the market which once you see them you may prefer. They are also bringing out new nappies, as are totsbots - and there is a lovely new pocket nappy called a mini la la (((patterns are gorgeous)))