SELECT COUNT(1) FROM [qdfSales] GROUP BY FORMAT([DocumentDate], 'dd.MM.yyyy')

SELECT
ts.UserName,
SUM(ts.SaleAmount) AS [Сумма сделок],
(SELECT COUNT(1) FROM (SELECT DocumentDate, UserName FROM tblSales GROUP BY DocumentDate, UserName) WHERE UserName = ts.UserName AND FORMAT(DocumentDate, 'mm.yyyy') = FORMAT(<FILTER1> , 'mm.yyyy')) AS [Количество отработанных дней в текущем месяце],
SUM(ts.SaleAmount) * 0.5 AS [Зарплата]
FROM tblSales AS ts
WHERE FORMAT(ts.DocumentDate, 'mm.yyyy') = FORMAT(<FILTER1> , 'mm.yyyy')
GROUP BY ts.UserName