Id say you need at least 25 nappies too, but they dont all have to be XS ones. If you have enough XS/Small to get you through the daytimes, then you can put on bullier ones at night that will fit longer (like onesize nappies etc). Unless you have quite a big nappy budget it doesnt really seem worth buying 20 XS nappies as they'll be grown out of so quick, it may be worth placing a wanted ad in the classifieds for some, or asking nappy companies if they'll do a discount when you buy more.
I know personally, I'd probably get 4 each of XS sandies, size 0 bimbles and size 0 kissaluvs, then another 15 size 1 nappies (wahm nappies, pockets, everything else really!) and then some onesize nappies to make up the rest. (yes, I am thinking about the newborn nappy stash for a LO who hasn't even been concieved yet
I had some mothercare flat shaped nappies for the newborn stage. They worked well and were super cheap because they were preloved! Mothercare dont do them anymore but you may be able to find similar ones. Terries, small prefolds and muslins also work well on tiny babies