How do I find the most frequent number from patient_id

An image of the table to retrieve the data from:

REM ADMISSION TABLE

INSERT INTO ADMISSION VALUES (205,101,'2/2/2011','HB',114,'P','21/2/2011');

What is "most frequent number"? Patient who has most admissions? If so, then

select patient_id
from (select patient_id,
             rank() over (order by count(*) desc) rnk
      from admission
      group by patient_id
     )
where rnk = 1;