Asalamualikum
Too much confusion these days regarding the number of units of Taraweeh... This extract gave some solid answers to this confusion. May Allah guide us to the right path. This is an extract from the book of Maulana Safdar Sahab.
http://eislamtherightpath.blogspot.com/2013/07/20-rakat-kyn-parhna-zaroori-hai-why-20.html
Nabeel Faruqui