Copy and paste issue from Excel to Keynote table
Your delimiter in numbers is set to a 'space', so it reads spaces in the sentence as being cell delimiter. Hence, each word goes into a separate cell. You need to change the delimiter to a comma or a tab. Then only values separated this chacter will fill new cells. To change delimiter (taken from question):

In Numbers you can use any separator when importing csv tables.
On the format sidebar, in the table tab, look for adjust Import settings. Inside it you have delimited options where you can even use custom delimiters