First, let's look at the Oregon Petition. They define "scientist" as anyone with a BS degree or better. They state, "This includes primarily those with BS, MS, or PhD degrees in science, engineering, or related disciplines." Thus, 31,000 is their numerator.
According to the US Census for 2000, 28 million people had bachelors degrees and 16 million had graduate or professional degrees. We'll safely assume that half of the bachelor degrees are BA's and not BS degrees. In 2000 that represented about 10% of the population. If the proportions hold today it leaves us with a total of 31 million people of the current US population of 312 million (Note: the Oregon Petition is limited to the US).
