Function h265_dpb_frames 

pub fn h265_dpb_frames(level: u32, width: u32, height: u32) -> u32

The reference frames an H.265 stream at level may hold for a width x height picture: the decoded picture buffer A.4.2 sizes (six pictures at the level’s full MaxLumaPs, more for a smaller picture) less the picture being decoded, at most REFERENCE_FRAMES.