Either of these will work. The second is better, but the first will show you how you can go about this when grouping by isn’t so straight-forward (or achievable).
SELECT
ISNULL(SUM(CASE WHEN OS_NAME = 'Linux' THEN 1 ELSE 0 END), 0) AS [Linux Servers],
ISNULL(SUM(CASE WHEN OS_NAME = 'Windows' THEN 1 ELSE 0 END), 0) AS [Windows Servers]
FROM YourTable
SELECT OS_NAME, COUNT(*)
FROM YourTable
GROUP BY OS_NAME
solved Count SQL Query [closed]