Thanks Mahendra for the reply. You can change the number of frequencies but that change only calculated frequencies in results. No matter number you put there, you get only 5 mode shape plots.
It is a known bug and it is reported in SPR 874599 - "Frequency study results do not add additional plots when the number of frequencies is increased".
You can check in the customer portal(login required)
Right-click on simulation study > Properties
Under Number of frequencies - Enter the desired number of frequencies.