I am trying to estimate the market for MindMapping software.

My question is: How many people use MindMapping software in the world today? 

If you can provide details on the various sectors (business, academic,
private) that would helpful as well and trends in the number of users,
that would be very helpful as well.

These are some of the major players in this market:
ConceptDraw http://conceptdraw.com/en/
Inspiration Software http://www.inspiration.com/home.cfm
MindMapper http://www.mindmapperusa.com/
Visual Mind http://www.visual-mind.com/
MindManager http://www.mindjet.com/eu/
Mindgenious http://www.mindgenius.com/