Looking at the authenticity of the Ahadiths ,
the Prophet -Peace Be Upon Him- Prayed only 11 Rak'ats and on the other narration 13 Rak'ats.
Though the Salafs and the people who came after his generations added some Rak'ats and as we can see now its already 20.
Are they doing Bid'Ah?
No , i believe that they know and understand more than we do the proofs.
Still the only Hadith that they hold on to is " SALATUL-LAYL MATHNA MATHNA" << Night Prayers are 2 and 2 <<
Also , it is not mentioned that theres a limit in the Rak'ats of Tarawih.
and as we all know TARAWIH since considered as night prayers.
And to my observation , you can only find 20 Rak'ats in the 2Masjids ,Haram-al-Makki and MADANI.
Im referring to the Masjids here in Saudi Arabia.
About the Athars and the opinion of the Salafs and the generations that came after them,
it only makes things handy but its not a form of a proof to hold on to.
"Qawlur-rijal yustadallu laha la yustadallu biha"
Words of men are treated as a guide not as a proof.
At the END sunnah is still the best which is 11 and 13 and i acknowledge the 20 for the stated HADITH..
--Just saying my part .. ALLAH Knows Best.