Six
You can go through that many in a day with a newborn I'm afraid. Great if you can get some second hand stuff as well as buying new. Do you know anyone who may have some stuff stashed in the loft? Offer to buy it from them....most people are glad to get rid of it if they don't plan on having anymore babies.
Don't spend a bomb, go to Asda, Primark etc and get them now while in the sale. If you're planning on having more kids get vests, sleepsuits and baby sleeping bags (if using them) in neutral so they can be re-used next time even if you have the opposite sex
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I'd just get newborn and 0-3 size for the time being and get other sizes a bit later as you will get a feel of how much you may need. I have at least 20 of each size vest, sleepsuits, bibs, muslins as well as outfits and clothes for baby, lots of socks and scratch mitts and at least two coats/snowsuits just in case baby is sick on one. I have tonnes coz I've got them left from previous babies but strongly advise more, rather than less otherwise the pressure to put on the washing machine every single day can be one more thing to worry about when you're a new mum.
A lot of people say not to get many newborn as they grow out of them quickly but my experience has been quite different. My babies have been quite small and 0-3 would have completely drowned them when newborn. In fact mine were at least 2 and nearer to 3months by the time they were in 0-3month size....the little munchkins.
It may be a case of playing it by ear but definitely see if you can get any hand me downs from friends and relatives