Good question but not sure there is a simple answer. I don't have a specific number that I use, but if I'm brainstorming, the supply (total # of results) is only one piece of my consideration. I also look at the
QUALITY of the results that show up. If Google says there are 4,400,000 sites on a specific phrase but most of the results on the first page are from sites like articlecity and about.com then that tells me there may be room for me.
When I created flat-stomach-exercises there weren't a lot of sites that had more than 2-3 articles on flattening the stomach (of course that's all changed now

) so that was also a consideration when I was brainstorming.
So basically you have to look at the whole picture. Supply and demand is helpful but also consider the quality of the results, your passion for the topic (if you're doing an affiliate/info site), etc.
Although I must admit, I'm probably not the best person to give advice on brainstorming because I hardly ever spend a lot of time using keyword tools, etc. Using my gut and researching the quality of results has been my strategy over the years and it's worked out pretty well.
I did spend more time brainstorming when I bought SBI! since I paid for it but I usually just close my eyes and hope for the best. (OK, that's a lie... I do keep my eyes open most of the time).