According to patch 6.5 notes, Blue/Red buff are intended to give 2 stacks.
They don't.
For people who are confused and think that they mean it gave 2 stacks from blue/red buff when devourer was initially released:
Devourer initial release in 4.20 (2 stacks from champ kills/assists, 1 from everything else): http://na.leagueoflegends.com/en/news/game-updates/patch/patch-420-notes#patch-jungle-item-enchantments Sated devourer rework 5.13 (2 stacks from rift scuttler, 5 stacks from dragon/baron, 1 from everything else): http://na.leagueoflegends.com/en/news/game-updates/patch/patch-513-notes#patch-items
It did not initially give 2 stacks from buffs, it never did. Just rift scuttler.