Bosk Banneret Treefolk spells and Shaman spells you cast cost 1 less to cast question

if i have Bosk Banneret on battlefield

it says
Treefolk spells and Shaman spells you cast cost 1 less to cast.

and I play a Guardian of Cloverdell which is both treefolk and shaman

does it cost 1 less or 2 less to play?

Also how about changelings do they cost 1 or 2 less to play?

1 less in both cases. Bosk Banneret (and its Banneret friends) has one ability that asks "Are you a Treefolk and/or Shaman spell?". If the answer's yes, it costs 1 less; if it's no, then it doesn't.

Compare it to Grand Arbiter Augustin IV, which has two separate abilities that can reduce a card's cost by 2.