Get values from column that appear more than X times (without pandas or external libraries)
The "no pandas or other external libraries" instruction was included in the homework question? ;-)

You need a group-by the CUST_ID and then select on the count in each group. 

Your question doesn't specify, but you tag with Python. Otherwise I would say to load the data into an SQL database or use the great CSV toolkit.

If you need to implement it using Python, the group-by is done by sorting on CUST_ID, and then going through the sorted data (with a for-loop, for example), counting the number of repeated CUST_ID's. When the CUST_ID changes, you check how often you have seen this CUST_ID. If greater than X, then you output this CUST_ID. Otherwise, reset your counter.