How many people spectate motocross in the USA?

That's impossible to say how many, I found a percentage of the market, this site will explain the nuances of monies, participation, etc., of the sport:

(an excerpt) Something else I would like to clarify; America has 58% (not 52%) of the global off-road market because it's the richest and biggest country in the world with very large spaces for riders. A bit like Australia that, with a population of only 19millions of citizens and huge spaces to ride has 10% of the world market (which in relation to population would eventually have the biggest off-road share of the world market). Here is the site:

• http://www.motocross.com/articles/article/news/20071217/giuseppe_luongo_s_response-6362.HTML