How to fire trigger only once after insert into statement execution (multiple insert)?

The trigger will be fired after each DML statement. You cannot change that. But you can insert multiple rows in a single statement:

INSERT INTO YourTable (Columns)
VALUES
    (Row 1),
    (Row 2),
    (Row 3)
;

-- Or:
INSERT INTO YourTable (Columns)
SELECT ColumnsFromOtherTable
FROM SomeOtherTable
WHERE ...;