counting top 5 repeated zipcodes out of thousands of entries

Use PivotTable.

Put Zip code into row label field. And in data field add Zip code again but as "Count of".

Then add Top 10 filter, set to Top 5, by item, Count of Zip

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I would approach it this way, say the column A is where the zip code list is and column G is blank.
step 1 - copy the zip code list and paste it in col G
step 2 - go to the data tab and highlight the col G list and use the remove duplicates function.
step 3 - beside the new list in col G use a countif in col H where =countif(A:A,G1) then double click for it to count all the values in col G.
step 4 - apply a filter to col G and H and filter on values in H descending.
Other alternative would be to use a rank function (accounting for duplicates) to col H then somewhere put #s 1 through 10 in empty cell then use an index match against the #s 1 through 10 and the rank field.
without a sample file it is hard to be more specific.
Hope that helps.