Q: How many changes of clothes do I need?
A: One set of clothes with several layers.
Think layers: Wear clothes in layers. Put on or take off layers according to weather.
Instead of bringing several outfits, just bring one set of clothes with different layers. Obviously, you may not need to bring every layer and you need to adjust your clothing for the expected weather.
Note: When looking for inexpensive backpacking clothes, look through the athletic clothing sections. You can find synthetic (polyester, nylon, acrylic) socks, underwear, wicking t-shirts, pants and shorts. Wal-mart carries the Starter Dri-star brand & K-mart has Athletech brand. Target sells Champion C-9 brand. Don't forget to check Goodwill for jogging suits, zip-off pants, etc.
Base Layer:
Underwear - Avoid cotton! Use synthetic fabrics. Athletic stretchy underwear (compression/biking shorts) are great. Bring 2 pair of underwear. Change before you go to bed & wash your dirty pair 200 feet from any stream every morning. They'll be dry & clean in no time.
Socks - Avoid cotton! Carry 3 pair of synthetic socks. Wear 1 pair, have a backup pair, sleep in the third pair. Alternate them & wash 1 pair 200 feet from any stream every morning. Note - The newer style BSA socks are 100% synthetic.
Inner Layer:
T-shirt - Avoid cotton! Any lightweight wicking t-shirt is good. If it needs washed, wash it 200 feet from any stream & put it back on.
Shorts or pants - Nylon or polyester. The newer Boy Scout zip off pants work great. Jeans are heavy, take forever to dry and won't insulate when wet. Say it with me guys, avoid cotton!
Mid Layer:
Long sleeve shirt & pants for mild weather & sun protection. Synthetic wicking fabric is best.
Insulating Layer:
Fleece shirt or jacket, longjohns, glove liners. Again, avoid cotton!
Outer Layer:
Rain gear, heavy jacket, snow pants, gloves, etc.
Headwear:
Ball cap, wide brimmed hat, bandana, or a buff all are great for keeping the sun off you. Avoid heavy non-breathable materials in hot weather. A wool beanie or stocking hat is good for cooler weather or to sleep in.
Footwear:
Hiking - Boots with good ankle support has always been the standard footwear, but lately many backpackers prefer to wear lightweight trail runners. Your choice.
Camp - A pair of flop-flops, crocs or water shoes are great for wearing in camp and for water crossings.