Well, if you have a projective variety you can always find a basis of $NS(X)\otimes \mathbf{R}$ as an $\mathbf{R}$-vector space only made by ample $\mathbf{R}$-divisors. This implies that every projective variety (say over $\mathbf{C}$),if you suitably choose the basis of the vector space we are speaking about, has ample cone $(\mathbf{R}^+)^{\rho}$.

The reference is in the first chapter of the book "Positivity in algebraic geometry I", by R. Lazarsfeld. I don't have the book now so that I cannot tell you exactly what is the paragraph, but I'm pretty sure it is in the first chapter and it is just an exercise. Does this answer your question?

Well, if you have a projective variety you can always find a basis of $NS(X)\otimes \mathbf{R}$ as an $\mathbf{R}$-vector space only made by ample $\mathbf{R}$-divisors. This implies that every projective variety (say over $\mathbf{C}$),if you suitably choose the basis of the vector space we are speaking about, has ample cone $(\mathbf{R}^+)^{\rho}$. [retracted in comment below]

The reference is in the first chapter of the book "Positivity in algebraic geometry I", by R. Lazarsfeld. I don't have the book now so that I cannot tell you exactly what is the paragraph, but I'm pretty sure it is in the first chapter and it is just an exercise. Does this answer your question?