بسم الله الرحمن الرحيم
Twenty rakah tarawih is proved from consensus of Sahaba and the Ummah has agreed upon it (ijma). It is sunnah muwakkidah and one who leaves it will be sinful. Eight rakah as tarawih is not proved in anyway. Thus if one offers only eight rakah then he will also have to offer remaining twelve rakah only then sunnah muwakkidah will be fulfilled and it is a sin to miss it. It is major sin to develop the habit of missing tarawih.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.