You ABSOLUTELY need more light in areas where there is ambient light than when you're out in the country. You do need some level of light in the country to see by - IMHO about 200 lumens is the minimum that I feel comfortable with riding at or nearly at speed with, but 400 or so is more comfortable.

As far as "how much is enough" - when car drivers stop saying "I didn't see him" then we've hit "enough".