PROC FREQ is, essentially, creating a summary report -- given your code. There are no "observations" coming out of PROC FREQ -- FREQ is producing a report that can have titles and footnotes -- but essentially, just a REPORT.
If you wanted to create an output dataset, using PROC FREQ, then you would need to investigate how the OUT= option works or how the ODS OUTPUT destination works to create an OUTPUT dataset.
PROC FREQ does support WHERE statement processing. So these would all be valid PROC FREQs:
proc freq data=sashelp.class;
where sex = 'F';
Also, I'm not sure I understand what you want. If you had this data (sorted in descending order) what would you envision:
1) only the first 12 with the highest values for num. In this scenario, you would pick off the first 12 observations from the detail data and either create a report or an output dataset;
2) add up all the values for num by id and THEN pick off the highest 12 -- for example, in this scenario, ID of A1 would have a value of 1014 for NUM (the sum of the 2 observations for ID=AI) -- this could be either a report or an output dataset
3) summarize all the values, find the IDs of the 12 highest and then go back and get ALL the observations for these highest IDs. In this scenario, you would have 2 observations for A1 on a report or in an output dataset: [pre]
. . .
Depending on the result you want, PROC FREQ may not be the path to follow.
Like Cynthia said you will need to make your own report. But this could help get you started. The out= data from PROC FREQ is in the order you desire and you can use OBS=12 data set option to subset the data.