Praise be to Allah.
The meaning of bid’ah
The root of the word bid’ah in Arabic means initiating something without any precedent. For example, Allah says (interpretation of the meaning):
“The Originator of the heavens and the earth…” [al-Baqarah 2:117]
In the terminology of Shari’ah, bid’ah means something that has been introduced into the religion of Allah that has no general or specific basis to support it.
For example, the innovated adhkar (dhikr) such as saying the name of Allah on its own (“Allah…Allah… Allah”) or the pronoun (“Hu… Hu… Hu…”, meaning “He… He… He…”). This is a new thing that has been innovated in the religion, intended as worship of Allah, but there is no evidence for this, either specific or general… so it is bid’ah .
Praying more than 8 rak’ahs in Tarawih
With regard to praying more than 8 rak’ahs in Tarawih , the Sunnah as reported in the sahih ahadith, and as the Prophet (peace and blessings of Allah be upon him) did regularly, is to pray eleven rak’ahs at night. He used to pray eight rak’ahs (saying Tasleem after each two rak’ahs in most cases), ending with two rak’ahs of shaf’ and one rak’ah of witr.
He sometimes prayed thirteen rak’ahs, as was narrated in the hadith of ‘Aishah and Ibn ‘Abbas (may Allah be pleased with them both). Doing more than this is permitted, but it is not Sunnah. What indicates that it is permissible is the sahih hadith in which the Prophet (peace and blessings of Allah be upon him) said: “Night prayer is two by two” – meaning two rak’ahs by two rak’ahs; he did not limit it to a set number.”
And Allah knows best.