Dear Tore,
As described in this forum topic: Learizing Baseline 5MW Wind Turbine with FAST, generator-DOF-induced rigid-body modes show up in MBC3 as a pair of zero-valued (or near-zero-valued) frequencies with +/- inf damping. Thus, this mode can add one more mode than you have DOFs enabled. I suspect this is why you are getting 23 modes for a 22-DOF system.
I’m not sure I understand your last question. It may help to paste your results into the forum.
Best regards,