If I have a table and data like this:
ID | Name | Group
1 Apple A
2 Boy A
3 Cat B
4 Dog C
5 Elep C
You need to aggregate the data first, this can be done using the GROUP BY clause:
SELECT Group, COUNT(*)
FROM table
GROUP BY Group
ORDER BY COUNT(*) DESC
The DESC keyword allows you to show the highest count first, ORDER BY by default orders in ascending order which would show the lowest count first.