To change the number of modes computed you need to edit the EIGRL card in the Nastran bulk data file. If you reference the Nastran Quck reference guide you will see that field 5 of the EIGRL card is called "ND" is the number of modes. You may also want to give a numerical range of eigenvalues on fields 3 and 4 which are V1 and V2, if you still aren't finding the modes you were looking for.
There is a table in the quick reference quide under EIGRL that gives all the different combinatiosn of V1, V2, and ND that could be used for a buckling or vibration eigenvalue extractions.
And if you are using FEMAP you can find all the NX Nastran Manuals under Help > NX Nastran