Merge and formatting of data from two tables
Table creation and sample data script:

CREATE TABLE #Slot
(
    [Day]   tinyint NOT NULL,
    [Time]  time(0) NOT NULL,
    Venue   char(4) NOT NULL,
    Free    smallint NOT NULL CHECK (Free &gt; 0),

    PRIMARY KEY ([Day], [Time], Venue)
);

INSERT #Slot
    ([Day], [Time], Venue, Free)
VALUES
    (1, '08:30', 'RM 1', 10),
    (1, '08:30', 'RM 2', 10),
    (1, '10:30', 'RM 1', 20),
    (1, '10:30', 'RM 2', 20),
    (2, '08:30', 'RM 1', 10),
    (2, '08:30', 'RM 2', 10),
    (2, '10:30', 'RM 1', 30),
    (2, '10:30', 'RM 2', 10);

CREATE TABLE #Booking
(
    [Day]   tinyint NOT NULL,
    [Time]  time(0) NOT NULL,
    Venue   char(4) NOT NULL,
    [User]  varchar(10) NOT NULL
);

INSERT #Booking
    ([Day], [Time], Venue, [User])
VALUES
    (1, '08:30', 'RM 1', 'Jill'),
    (1, '08:30', 'RM 2', 'Jill'),
    (1, '08:30', 'RM 1', 'Jack'),
    (1, '08:30', 'RM 1', 'Mary'),
    (1, '08:30', 'RM 2', 'Mary'),
    (1, '08:30', 'RM 2', 'Jill'),
    (2, '10:30', 'RM 1', 'Ken'),
    (2, '10:30', 'RM 1', 'Ken');


Query:

SELECT
    s.[Day],
    s.[Time],
    s.Venue,
    s.Free,
    Used =
    (
        SELECT COUNT_BIG(*)
        FROM #Booking AS b
        WHERE
            b.[Day] = s.[Day]
            AND b.[Time] = s.[Time]
            AND b.Venue = s.Venue
    )
FROM #Slot AS s
ORDER BY
    s.[Day],
    s.[Time],
    s.Venue;


Output:

╔═════╦══════════╦═══════╦══════╦══════╗
║ Day ║   Time   ║ Venue ║ Free ║ Used ║
╠═════╬══════════╬═══════╬══════╬══════╣
║   1 ║ 08:30:00 ║ RM 1  ║   10 ║    3 ║
║   1 ║ 08:30:00 ║ RM 2  ║   10 ║    3 ║
║   1 ║ 10:30:00 ║ RM 1  ║   20 ║    0 ║
║   1 ║ 10:30:00 ║ RM 2  ║   20 ║    0 ║
║   2 ║ 08:30:00 ║ RM 1  ║   10 ║    0 ║
║   2 ║ 08:30:00 ║ RM 2  ║   10 ║    0 ║
║   2 ║ 10:30:00 ║ RM 1  ║   30 ║    2 ║
║   2 ║ 10:30:00 ║ RM 2  ║   10 ║    0 ║
╚═════╩══════════╩═══════╩══════╩══════╝