You can wear about half as much as every other person you'll meet will be wearing. I don't understand these people and their arctic expeditionary parkas - it's 40 degrees and breezy, not -20 and raining sideways.
That's what I'm hoping for. I have enough to wear for my morning commute, which sometimes starts around 40 degrees. That chilly commute is probably the ONLY reason I won't be one of those bundled up tourist wimps this time.