Few things to think about first. What's your budget like? Do you want future kids? When is your baby due?
Budget - This is key. Tbh you can get away with buying baby clothes from anywhere. It's all good. DC2 has enjoyed wearing stuff from Poundland, Primark, Tesco, Sainsbury's, Boots, M&S, and Next over his 10 months of life so far. I will say though, that the more expensive quality stuff washes better, and feels better. I love Next, but given in the early months, they will literally only wear it for, at most, a couple of months, then grow out of it. Which ties into...
Future kids - Quite honestly, for the first 3-6 months, my two lived in vests and baby grows. No faffing about with outfits. So I bought neutral high quality stuff, so that even though it doesn't last long on one child, it'll get multiple uses out of it. All babies look adorable in white. As they get older, and start to wear gendered stuff, then I start getting basics from supermarkets and Primark. Might only be for the one kid (I have a girl and a boy so far), so no point going mad. As they get older and clothes start lasting more than a year, then I'll look at the more expensive stuff again.
Baby's due date - Something to think about is what season will they be born in, and at what stage will they be for the rest of the year. No point buying a lovely thick woolly 6-9mo outfit, if it's going to be summer at that stage. Ditto for sun dresses and shorts in the depths of winter.
As for what you'll need, I would say at least 10 vests for each stage. In the early months 0-6 months, if you have a refluxy/poonami baby, you can always buy more. If it's hot, it's all they'll need to wear. I love vests. Baby grows, the same amount as vests as a rule, though if maybe hold off buying them in the size your baby will be in the summer months. If we have another scorcher like last year, you might never need them. As I said earlier, I tend not to do outfits until they're 6 months old.
As for other things light cardigans are useful. Socks go on tiny feet, then magically work their way off. Up to you if you want to bother. Mine get shoes when they go to nursery, or they walk. Whichever happens first. I got a pram suit for the winter months, as I walk everywhere a lot. Less useful if you regularly need to use the car. Otherwise, I find that friends and family tend to go a bit nuts buying baby clothes, so I just need to fill in the gaps.
Hope that helps a little.