It depends on a few things, the school and their rules, how messy your DC is, whether you want pretty options or just the basics, but for my DD I bought:
5 x white polo shirts
4 x grey pinafores
10 x tights (grey/school colour)
5 x white socks with school colour trim
3 x school-branded cardigans
Black school shoes
And for PE:
School colour shorts
School-branded PE t-shirt
Black jogging bottoms
School-branded PE fleece
Black socks
Black trainers
I also bought hair accessories and bows in the school colour, not necessary really but it makes her look smart. Well, at least it does when she leaves in the morning, less so when she comes home.
This term, she's in summer dresses so I've bought 2 of those and will probably need 2 more.
One day a week, she has forest school and wears own clothes to school.
In school they have a school book bag, a PE bag (school-branded) and a forest school bag with their wellies and waterproofs. Parents are specifically asked not to send in backpacks etc. as there's no room in the cloakroom.