This sets a fixed number of equilibrium iterations in each time step (for time steps of the control_timestep record with the same index). If number_of_iterations is 1 then the time stepping is completely Euler explicit. For larger values, the time stepping becomes implicit. For many iterations, the time stepping is Euler implicit. Typically, number_of_iterations is 1,2 or 3. Default number_of_iterations is 2.
As an alternative, you can use control_timestep_iterations_automatic.