I bought around a dozen 0-3 mo onesies and a dozen 3-6 mo onesies. I bought a pair of jeans, a few cute shirts and maybe 4 dressy dresses in each size and then the rest was sleepers and play clothes (a few one piece rompers and lots of comfy stretchy pants/leggings to be worn with onesies.) Dd is 10 months and in 12 month clothes and we can still stretch those 3-6 mo onesies with no problem. I don't like to use onesies once they are standing/walking so I didn't buy any after 6 months but the ones we have still work fine for occasional use at bedtime or whatever.
In terms of clothes now, I have 4 thrift stores that I get most of dd's clothes from, so I mainly buy separates vs actual matchy outfits. Right now she has 3 dresses, 2 skirts, 5 pajamas, 4 jeans, 6 or 7 stretchy pants/leggings, a few cute tees, and some assorted hoodies and fleece tops. I plan to buy her probably 6 or so more shirts and a couple more dressy dresses (we dress up twice a week so this is important for us). When we go out to run errands or something she usually is dressed like me: hoodie, tee and jeans/ sneakers, or a dress/long shirt with leggings. She has a few pairs of cotton pants that are more like sweats that she doesn't really leave the house in.
I actually find that the cost for me is in accessorizing. Millions of hairclips, barrettes, colored elastics, earrings and shoes to match every outfit, I'm having a blast but her dad's wallet is not, lol. He was just complaining today because I bought a new pack of hairbows in target.
On a side note, not to hijack the thread, but does anyone know where they sell 12 mo pants that fit skinny girls, or have the adjustable waistband?