We shall work on the following problem: given a discrete subgroup \Lambda of PGL(d,\R) how many elements g\in\Lambda are there whose operator norm is smaller than $t$ when $t$ goes to infinity, for a fixed euclidean norm on $R^d$. We will give a precise answer for a class of groups which contains, for example, Hitchin representations of surface groups and groups dividing a strictly convex (open) set of $P(R^d).$