Two
Two.
There are no triangles.
21
Triangles do not have to have any obtuse angles but they can have one.
A triangle can have at most one obtuse angle, but triangles do not necessarily have any obtuse angles.
There are 8 triangles.
The least number of obtuse triangles, if all possible triangles are drawn for n points in a plane, is zero. If all the n points lie in a straight line, no triangles are possible and so no obtuse triangles are able to be drawn; thus for any number n, there is a possibility that no obtuse triangles can be drawn, so the least possible number of obtuse triangles drawn is zero.
All triangles have 3 sides.
4
9
9