Not sure about a capsule wardrobe, or 'rules', but this is what works for me:
I work from home so don't require office clothes, etc. But I do like to dress up now and again. I also don't have a riveting social life!
I find that casual is what I tend to wear most, so when I do buy a dressy thing, or a special bag, I will put some thought into it, hoping it will last.
Pants
3 pairs jeans: indigo, black and mid blue. One of them was expensive.
2 pairs of patterned jeans, both inexpensive but a great fit.
1 pair wool pants and 1 pair of leggings for cold days.
Skirts
1 cord skirt, above knee (H&M).
1 navy cotton mini (APC).
1 knee length indigo denim a-line (Oasis).
1 shortish wool above knee (APC).
Tops
1 peach silk shirt (aubin&Wills).
2 patterned cotton blouses (Isabel Marant/Vanessa Bruno).
2 silk vests (Aubin&Wills).
2 check shirts (Abercrombie/APC)
3 tshirts (APC/Whistles/Oasis)
1 breton (Aubin&Wills).
Dresses
1 smart lace dress above knee (Topshop).
1 check shirt dress, above knee, cotton (APC).
1 Silk dress (APC).
Knitwear
2 long cardi's (Massimo Dutti/Zara).
1 cashmere round neck jumper (LaRedoute).
1 grey slouchy merino (Boden).
1 beige cable jumper (Oysho).
1 navy merino jumper (Uniqlo, shit quality!)
Coats
1 long navy wool coat (Zara).
1 navy quilted jacket (Oasis).
1 trench coat (Burberry).
1 cropped boucle jacket (Zara).
Bags
1 burgundy tote (Jaeger sale).
1 smart small bag (Mulberry Lily).
1 tan satchel (Massimo Dutti).
Shoes
2 suede ankle boots (APC/Office)
2 patent heels (K Geiger/ Topshop)
1 Nike Dunks in black.
1 white converse, low.
2 leather summer sandals (APC/ Geiger).
Sorry the list is long. This is all that I own and it feels like too much, but it all gets worn. Not sure if I look like a princess or a pauper tho!