How can I know the number of k-points that I should use for my system?

As a rule of thumb for choosing the initial k-point sampling one should consider the product between the number of k-points in a given direction (k_{i}) and the length in Angstrom if the basis vector in that direction (a_{i}) and take this product as Lithe initial number of k-points:

- k
_{i}a_{i}~ 30 Å, for d band metals - k
_{i}a_{i}~ 25 Å, for simple metals - k
_{i}a_{i}~ 20 Å, for semiconductor - k
_{i}a_{i}~ 15 Å, for insulator

Remember that the convergence of this parameter should be checked.

You may find here a helpful SIESTA tutorial about converging the simulations, check point 1 (Convergence of electronic and structural properties of a metal with respect to the k-point sampling: bulk Al) and point 2 (Convergence of energetic and structural properties of an insulator with respect to the k-point sampling: bulk BaTiO_{3}).