I have a newborn and find 4-6 pieces of shortsleeved onesies are not quite enough, unless you want to wash a small quantity of clothes every day. Newborns' nappies tend to leak a lot because they sometimes don't fit very well when the legs are skinny, and they often spit up their feeds a lot too, while their stomachs get used to functioning. I would recommend 8 pieces, but you might not need so many for older babies. But don't forget people will buy some for you, so 4-6 for you to buy in advance will probably be enough!
The newborn is my second baby - the other one was born in Dec and we mostly dressed her in two layers of onesies, shortsleeved one underneath long sleeved one with legs & feet in, so that the latter can be stripped off when it gets warm. That time of year, when you are in the sun and sheltered from the wind it can be quite hot, but chilly in the wind and shade. Ditto sunny rooms of your flat. So you can be constantly putting layers on and off.
One thing I would recommend is not buying too much in bigger sizes, because every baby grows at different rates and it's impossible to judge what size your baby will be after 6 months. My toddler is average height but slim with short arms and legs, and she has almost never fitted things with her age label on them. If you plan on using cloth nappies then you need "big bum" things!
Also, if you are shopping in the markets, first measure how big a 0-3 month onesie should be from a reputable brand, then take a tape measure with you shopping, because I have found that a lot of things are mis-sized in the markets. I think that's why a lot of perfectly good branded clothes end up in the markets, because they have the wrong label on them and it was too much trouble for the manufactuers to re-label them.