Below are some methods for importing CSV data into a Numbers document. I misread your problem thinking this was it but it sounds like it imported correctly; it is the process of copying it from the newly created document to your other document that is the problem. See my next post about that.
A CSV file in Finder that is opened using Numbers will create a new Numbers document with one table on one sheet that has the correct number of columns and rows for all the data and no extra rows or columns. It does not involve any copying or pasting.
A CSV file that is drag-dropped to an empty place on a Numbers sheet will create a new table that has the correct number of columns and rows for all the data and no extra rows or columns. It does not involve any copying or pasting.
A CSV file that is drag-dropped from Finder onto an existing Numbers table will import the data into the table and will add the necessary number of columns and rows. If drag-dropped onto a table that is already populated with data, the new CSV data will be placed below all existing data. No copying or pasting is involved.
If a CSV file in Finder is copied then pasted into a single selected cell in a Numbers table, the CSV file will be pasted into the table starting at that point. The table will expand if necessary.
If a CSV file in Finder is copied then pasted into a Numbers table where all the cells have been selected, the CSV file will be pasted into the table and, if there are additional empty cells in the table, data from the CSV file will get repeated to fill the entire table.
If a CSV is opened in TextEdit and the contents copied then pasted into a selected cell in a Numbers table (selected with a single click, not two), it will usually import it as "table data" and put everything in the correct columns. But sometimes it does not recognize it as "table data" and may use only one column.
I'm sure there are other variations. If one of these methods does not work, it may be a problem with the CSV file.