In the Modal Analysis user has the objective of obtaining the frequencies and shapes of each mode. As the number of eigenvalues to be calculated has to be given it is the Cumulative Effective Mass Participation Factor (CEMPF) in each direction that is used to indicate up to how many modes should the user go. In general, the most important are the natural frequencies, the lowest, with the most effective mass participation value; tough, for the analysis of bridge structures users usually need to get up to at least 90% Cumulative Mass Participation Factors for the directions considered important. Therefore, it is an interactive solution and a user engineering judgment of approximately how many eigenvalues are necessary for receiving the level of mass participation in each direction
If the default * file name is used in the Action Eigen in the Stage, for the output of mass participation, the user just has to search in the eigen.lst results report list file for a paragraph like the one shown in the image below. In the example below, only 10 modes would be good enough for both longitudinal and lateral directions but not for the vertical direction.
Additionally, the tolerance value for determining the accuracy of the calculated eigenvalues and the Modal Analysis methods, Eigen or Ritz, are specified in the Recalculate Analyzer pad in the menu at Advanced Options \ Dynamics.
More info at the link and the training 5 for Seismic: