Usually, the only things I carry "extra" are socks and a cotton t-shirt to sleep in. Everything else is just one... layers are about as follows:
Normal clothing is pants, synth underwear, synth t-shirt, nylon long sleeve shirt, tilly hat, knit hat, rain poncho, wool socks, and merrill moab ventilator shoes.
When it's a little colder, add a sweater, long johns, a vest (thin or thick), some sort of outer wind/waterproof layer, and maybe muck boots vs shoes. Maybe swap the tilly for a ballcap (knit hat fits over it better).
When it's a lot colder, maybe add an outer layer of "field pants", add a liner for the pants and jacket, change footgear to thicker socks and felt-lined boots.
I generally don't bring a lot of spare clothes just to have them 'clean'. I wash as I go.