Try the below queries -
SELECT COUNT(*) FROM tblPlan WHERE CONVERT(DATE,StartDate,103) >= CONVERT(DATE,@StartDate,103) AND CONVERT(DATE,EndDate,103) <= CONVERT(DATE,@EndDate,103)
SELECT COUNT(*) FROM tblPlan WHERE CONVERT(VARCHAR,StartDate,103) >= CONVERT(VARCHAR,@StartDate,103) AND CONVERT(VARCHAR,EndDate,103) <= CONVERT(VARCHAR,@EndDate,103)