Hi Ismael,
Currently, for the general flexible plate, the number of nodes/elements depends on the complexity(holes, grooves, ...) of the geometry of the plate's mid-surface and the number of frames added to it. Adding more frames to the plate adds more nodes to the plate's FE solution; in the meantime, please note that adding more nodes will slow down the solution.
The current plate's FE solution solves for the physical states of all the plate's nodes(x,y,z,theta_x, Theta_y) without any modal reduction, so the number of normal modes is not required for the current solution.The good thing is that the modally reduced solution that allows the user to control the number of normal modes to include in the dynamic solution is expected to be available for different flexible blocks in a coming release.
Many thanks,
Ahmed