How to insert numerous data into temporary table?
You can use VALUES (...), (...):

INSERT INTO table(colA, colN, ...) VALUES
    (col1A, col1B, ...)
    , ...
    , (colnA, colnB, ...)


With @In:

DECLARE @In TABLE (Col CHAR(20))
INSERT INTO @In VALUES 
    ('value1')
    , ('value2')
    , ...
    , ('value1000')


It will insert X rows all at once. GO is not needed. Variables declared before GO don't exist anymore after GO.