Today on my way in I was wearing a shirt, jumper, big woolly hoody and a raincoat, but only quite thin trousers on my bottom half. Not including socks and pants obviously. Tights under a skirt don't count either, unless maybe it's an ankle length jobby.
That's a 4:1 ratio. I might have worn thermals under the trousers if our office wasn't always so bloody hot, so I'll say that 3 is the most amount I would wear without a second on the bottom. But in most cases it's 4.