I think it really depends on the individual child. We have figured out our perfect amount of uniform for us now but it has taken a few years.
I buy 4 shirts, 4 bottoms (1 skirt, 1 culottes and 2 trousers), 2 cardigans and 1 jumper. (And eleventy billion pairs of school socks as they mysteriously vanish into skinny air in this house). If she was a boy I'd go with 3 bottoms, we only get 4 as she likes to have options but trousers are the main favourite.
The question of shirts vs polo shirts and jumper vs sweatshirt, again it's personal choice. I would say initially get the option of both but only buy 1 of each and see what your DS likes the best. After a few weeks you know what option he is asking for the most and you can buy more then.
We had a similar option when starting school (now changed to shirt and tie only
) and found DD liked the shirt and tie over polo shirts the first year. This changed the following year when she liked polo shirts better.
Sweatshirts, jumpers and cardigans are handy but most schools are very warm so they get taken off and stuffed in bags more often than not. We could probably get by with one but I like 2 for those days it gets let in school or she spills toothpaste down it in the morning or if you are a lazy parent like me, you take it out the bag the next morning to find a crusty yoghurt stain down the front and don't have time to wash it.
For skinny children most shops do adjustable waistbands on trousers (Asda appears to be the exception along with 1/2 of the sainsburys range). M&S tend to be wide fitting in shirts and size bigger in general that the other shops (as did he now defunct BHS). Skinny DD always looks swamped in M&S stuff. I have found Debenhams trousers fit slimmer as do Asda blouses (not sure about the polos or standard shirts). Next stuff is good quality and they offer slim fit in a lot of things which is great BUT if you have a dog or a cat you might be driven mad as a lot of thier uniform has that type of fabric that attracts hair and fluff - dog hair magnet fabric as it's know in our house.