Under specific circumstances, magnetic flux penetrates into superconducting thin films as dendritic flux jumps. The phenomenon has a thermomagnetic origin, where flux motion generates heat that suppresses flux pining and facilitates further flux motion. We have studied the thickness influence on the flux stability for very thin Nb films, 20, 40, 60, and 80 nm, through dc-magnetomerry. The thicker the film; the higher is the threshold field where instabilities first take place. Due to the demagnetizing factor in a perpendicular geometry, the effective magnetic field at the border of the film is largely amplified. For thin specimens, a linear dependence between the threshold field and the thickness is expected and has been actually observed. When normalized by the sample aspect ratio, the effective threshold magnetic field is nearly the same for all specimens studied.