Mahayana Buddhism is the most popular and alongside with Ancestor Worship, Taoism and Confucianism as one which is the traditional faith of general Han Chinese people (about 15% of total population of Brunei).
From many estimates of different sources but the most common figure today is 14% of Brunei's population are Buddhists.
And Buddhism is also the second largest religion after state religion of Brunei: Islam.
References
- CIA FactBook - Brunei [1]
- The US State Department's International Religious Freedom Report 2006 - Brunei [2]
- Religious Freedom page - Brunei [3]
- Religious Intelligence - Brunei[4]