Thank You for Your Reply,

  Yess , My first natural frequency 2800 HZ , whereas PSD input is 20 to 2000 HZ. As you said if i cnrease the number of modes it does increase the Mass ratio. But I want to ask whether doing Random vibration in this case is give correct result or just any arbitrary values? 

Also can you please tell me how can I verify that wheather my Modal Analysis results are correct or Not?