Newton-Okounkov polygons with a small number of vertices and Picard number

Newton-Okounkov bodies serve as a bridge between algebraic geometry and convex geometry, enabling the application of combinatorial and geometric methods to the study of linear systems on algebraic varieties. This paper contributes to understanding the algebro-geometric information encoded in the collection of all Newton-Okounkov bodies on a given surface, focusing on Newton-Okounkov polygons with few vertices and on elliptic K3 surfaces. Let S be an algebraic surface and mv(S) be the maximum number of vertices of the Newton-Okounkov bodies of S. We prove that mv(S) = 4 if and only if its Picard number ρ(S) is at least 2 and S contains no negative irreducible curve. Additionally, if S contains a negative curve, then ρ(S) = 2 if and only if mv(S) = 5. Furthermore, we provide an example involving two elliptic K3 surfaces to demonstrate that when ρ(S) \geq 3, mv(S) no longer determines the Picard number ρ(S).