Praise be to Allah.
The root of the word bid’ah in Arabic means initiating something without any precedent. For example, Allaah says (interpretation of the meaning):
“The Originator of the heavens and the earth…” [al-Baqarah 2:117]
In the terminology of sharee’ah, bid’ah means something that has been introduced into the religion of Allaah that has no general or specific basis to support it.
For example, the innovated adhkaar (dhikr) such as saying the name of Allaah on its own (“Allaah…Allaah… Allaah”) 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 Allaah, but there is no evidence for this, either specific or general… so it is bid’ah.
With regard to praying more than 8 rak’ahs in Taraweeh, the Sunnah as reported in the saheeh ahaadeeth, and as the Prophet (peace and blessings of Allaah 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 hadeeth of ‘Aa’ishah and Ibn ‘Abbaas (may Allaah be pleased with them both). Doing more than this is permitted, but it is not Sunnah. What indicates that it is permissible is the saheeh hadeeth in which the Prophet (peace and blessings of Allaah 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 Allaah knows best.