Don't know the exact syntax offhand, but play around with the SQL GROUP BY functions. Something like:
SELECT number, COUNT(number) AS num_count FROM tbl_name GROUP BY number ORDER BY num_count DESC LIMIT 5;
This adds an extra column to the result set, called num_count. For each number in the database, num_count will contain the number of times it appears; you can then ORDER BY num_count and retrieve however many results you need.
Just replace tbl_name with your table's name and number with the appropriate column name.