SET dateformat dmy;
CREATE TABLE SampleData (
   [Group] Varchar(20),
   [StartDate] Date,
   [EndDate] Date
);
INSERT INTO SampleData ([Group], [StartDate], [EndDate])
VALUES ('1', '01-01-2020', '01-07-2020'), 
       ('1', '01-08-2020', '01-07-2021'), 
       ('2', '01-02-2020', '01-03-2020'), 
       ('2', '01-09-2020', '01-12-2020');
SELECT * FROM SampleData
DECLARE @MinDate DATE = (SELECT MIN(StartDate) FROM SampleData)
DECLARE @MaxDate DATE = (SELECT MAX(EndDate) FROM SampleData)
;WITH n AS 
(
    SELECT 0 AS n
    UNION ALL
    SELECT n + 1 
    FROM n 
    WHERE n + 1 <= DATEDIFF(DAY, @MinDate, @MaxDate)
)
SELECT DATEADD(DAY, n, @MinDate) AS Date 
INTO #Calendar
FROM n 
OPTION (MAXRECURSION 0)
CREATE TABLE #Result
(
    Date DATE,
    [Group] VARCHAR(20),
    Counted INT,
    CumulativeCount INT,
    ResetDate DATE
)
DECLARE @Group VARCHAR(20), @Date DATE
DECLARE GroupCursor CURSOR FOR 
SELECT DISTINCT [Group] FROM SampleData
OPEN GroupCursor
FETCH NEXT FROM GroupCursor INTO @Group
WHILE @@FETCH_STATUS = 0
BEGIN
    DECLARE DateCursor CURSOR FOR 
    SELECT Date FROM #Calendar ORDER BY Date
    OPEN DateCursor
    FETCH NEXT FROM DateCursor INTO @Date
    DECLARE @CumulativeCount INT = 0, @ResetDate DATE = NULL
    WHILE @@FETCH_STATUS = 0
    BEGIN
        DECLARE @Counted INT = 
        (
            SELECT CASE WHEN COUNT(*) > 0 THEN 1 ELSE 0 END
            FROM SampleData
            WHERE [Group] = @Group AND @Date BETWEEN StartDate AND EndDate
        )
        IF @Counted = 1 AND (@CumulativeCount = 0 OR DATEDIFF(DAY, @ResetDate, @Date) > 365) 
        BEGIN
            SET @CumulativeCount = 1
            SET @ResetDate = @Date
        END
        ELSE IF @Counted = 1 AND @CumulativeCount < 100
            SET @CumulativeCount = @CumulativeCount + 1
        ELSE IF @Counted = 1 AND @CumulativeCount >= 100
            SET @Counted = 0
        INSERT INTO #Result (Date, [Group], Counted, CumulativeCount, ResetDate)
        VALUES (@Date, @Group, @Counted, @CumulativeCount, @ResetDate)
        FETCH NEXT FROM DateCursor INTO @Date
    END
    CLOSE DateCursor
    DEALLOCATE DateCursor
    FETCH NEXT FROM GroupCursor INTO @Group
END
CLOSE GroupCursor
DEALLOCATE GroupCursor
SELECT 
    [Group],
    FORMAT(Date, 'yyyyMM') AS YearMonth,
    SUM(Counted) AS CountDays
FROM #Result
GROUP BY [Group], FORMAT(Date, 'yyyyMM')
ORDER BY [Group], YearMonth