How many inversion drums, or other relining inversion vessels are sold
in the world annually. An inversion drum is a device that
invert/install a flexible liner into a pipe/drain/sewer. The liner is
impregnated with an epoxy and the liner is then cured forming a new
pipe inside the existing pipe. This is a pipe repair method without
digging. Here are two samples of inversion vessels.

http://www.epros.co.uk/ click on products then DrainLiner

http://www.brawoliner.de/02.produkte/06.inversions-t/

I am happy with educated guess as long as i know on what basis the gues is made

You can get number of units sold by dividing total global revenue by
average unit price.  An rough estimate of average unit price is easy
to find on your own.  Revenue is public information for publicly
traded companies.  Private company revenue can be estimated from data
on publicly traded companies, using any information you have about the
private company's size.  The more of a commodity market this business
is in, the more accurate estimations will be.  Please comment if you
need further details on getting the necessary information.