I do think it really depends on the child. As you have a DS it should be easier as you have far less options - trousers for most of the year I suspect and shorts for hot weather.
To start with 3 bottoms and 3 tops should be ok. You can go from there and buy more at a later date if needed. Shoes are usually the biggest issue. If your DS is tough on shoes then better quality/ more expensive will be the best option. Some people do go down the buy cheap and replace often route which over a year might work out more expensive (unless your Child’s feet grow quickly and you need 2/3 new pairs of expensive shoes so in that case it will cost more than the cheap and often route.)
DD is 8 now and has 3 short sleeve blouses and 2 long sleeve, 2 cardigans and far more bottoms.
She likes trousers for colder weather so we have 3 of them (although did have more at one point when moving between 2 sizes - 6-7 was just fitting but bordering on a bit too short and the 7-8 was just a touch too long). In addition to the 3 pairs of trousers she has 1 skirt, 2 culottes and 1 pair of city shorts. She goes between these in the nicer weather. We also have 2 summer dresses (one of which is more like a playsuit).
Socks are also another thing for girls. Long socks in grey, short socks in black for winter and both short and long in white for the summer. She also has some tights floating around from last year that still fit but don’t get used much as she rarely does the tights and skirt combo.
Next year I think I will go for 5 short sleeve blouses and leave the long sleeves altogether as they got hardly any use. I will stick with the 3 ‘seasonal’ bottoms and 2 cardigans as this works well for us.
My DDs friend has 2 sets of uniform and is frequently in non uniform items (ie: leggings in place of trousers or a hoodie in place of cardigan) as her dad didn’t have time to get things washed and dried on time (she lives with her dad full time). They don’t have a tumble dryer so drying can be a problem in the winter.
Quality wise I’ve found the supermarkets are much the same, M&S more hard wearing but appear to be made for larger kids (ie:not for the slight of build) the fabrics are also stiffer so not great for children who have issues with how clothes feel. I’ve found Debenhams stuff to be good but limited for colours (particularly if you need blue shirts or navy trousers) - ditto next (although the range of blue shirts is better).