Your best bet is a mintel report or bikebiz - but it will be an approximation based on mountain bikes sold which im sure you know in no way relates to the amount of people who actually mountain bike.
And whats your definition of mountain biking - trail cycling, racing, club riding, once a week, year, month?

?
Whats the difference between leisure cycling and mountain biking - or those that ride their bikes in to the hills to climb a mountain.
You need to get a bit more specific for a business plan mate.
Maybe assume that bikes over X amount are going to be used more seriously therefore could be classed as mountain bikers - ooh looks tricky........