Mahayana Buddhism is the most popular and alongside 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
Buddhism in Brunei Wikipedia(Text) CC BY-SA