WC-level stadiums are not the same thing as normal stadiums, i.e. a stadium for 20k people is not ok. They are planning to build 9 and expand 3, and they are actually going to dismantle part of them after they are used. Wikipedia has some stuff[0][1].

[0] " Local organizers are planning to build 9 new stadiums and expand 3 existing stadiums for this event." http://en.wikipedia.org/wiki/Qatar#Football

[1] http://en.wikipedia.org/wiki/Qatar_2022_FIFA_World_Cup_bid#P...