The word seemion occurs 70 times in the Christian Greek scriptures, according to Bagster’s ‘Englishman’s Greek Concordance of the New Testament’. There it is variously rendered as sign / signs; miracle / miracles (19 times); token (once in 2 Thess. 3:17). The vast majority is ‘sign/s’ = 50 times. But you only ask for the meaning in the Book of the Revelation. According to Bagster, it is ‘wonder’ three times, ‘miracles’ three times, and ‘sign’ once (in Rev. 15:1).
According to the book below, the word occurs seven times (as you have noted) and some would see symbolic significance in that number, in and of itself. The Greek word means ‘a sign; mark; or signal’. It is the word from which ‘semaphore’ is derived, indicating signs made with flags held in various positions equivalent to each letter of the English alphabet: sign language. And what is the Book of the Revelation if not revealing God’s sign language to those looking out for spiritual sign-posts and meanings? Now I quote from this book on the point you ask about, why the KJV renders seemion as ‘sign’ once, ‘wonder’ twice and ‘miracles’ three times :
“However, of these seven occurrences – as if a perversity took the
translators to confuse the reader – this simple Greek word, having so
clear an English equivalent in the word ‘sign’, has been translated
‘miracle’ three times; ‘wonder’ three times; and ‘sign’ but once. Any
schoolboy can see that ‘miracle’ and ‘wonder’ are not the same thing
as ‘sign’, being far more excessive in description… Returning to the
first ‘great sign’, that of Revelation 12:1, it is necessary to state
the obvious: this is a sign. It is not the thing signified. It is not
the reality; it is an allegory. It is not the substance: it is a
visionary shadow. It cannot be over stressed: the rules of
interpretation common to the Book of the Revelation apply here just as
much as anywhere else. This is so obvious. The great sign of a woman
appearing in the heavens? Clothed with the sun; the moon under her
feet; twelve stars her diadem? Bearing a child in the visible heaven?
If anything is a figure, this is a figure… It is not the first time
that certain of these details have appeared in a vision – or at least
a visionary dream – before. Something like this occurred some nineteen
hundred years before the birth of Christ. And it occurred to Joseph.
[Then quotes Genesis 37:9-11, Jacob depicted as the sun; Joseph’s
mother as the moon; his eleven brethren as the eleven stars, of which
he himself therefore – a figure of Christ – was the twelfth.] pp276 –
He goes on to point out that the virgin Mary bore the Son in a manger in Bethlehem, not in the visible heavens. Her child was not immediately caught up to God’s throne in heaven, but taken down the road to Egypt. Mary did not retire to the wilderness until the coming again of Christ. She did not live there, bearing generation upon generation of “the remnant of her seed” (Rev. 12:14-17). Yet the sign of that heavenly woman assuredly “includes Mary, just as it does Eve, but the graphic imagery of the mysterious vision vastly transcends the individual.” And, because Christ preceded the church, that heavenly woman could not have borne him when she, the Church, did not so much exist until the ascension. The Church in Rev. chapter 12 appears as ‘the remnant of her seed’. But if the ecclesia be the remnant of her seed, then she, the mother of that seed, cannot possibly be the same as that ecclesia which she bears. Texts such as Gen. 3:15, 1 Cor. 15:23, Gal. 4:22-28, Col. 1:15 and Heb. 1:6 are used to work out the meaning of this great sign, thus scripture interprets scripture (pp281-283).
“She is the concept that bore Christ and his seed, called heavenly
Jerusalem and spiritual Zion – Psalm 87:5-6 (p288).
There is no need to go into more such details for the other six occurrences of this word seemion in Revelation. Examination shows the difference between a sign and a miracle. What needs to be stressed is how misleading it is for us to muddle a heavenly sign up with a miracle, for miracles may – or may not – be from God. But the language of the Revelation gives us the signs from heaven that spell out the danger of thinking the evil one’s agents are performing miracles of God, when they are actually misleading and deceiving the whole world. They perform signs and not miracles, because they are not visible entities on earth. They are agencies, not individuals. Those who pay attention to the sign-language of God will spot the difference between his signs and the counterfeit signs of the red dragon’s agencies at work amongst humanity in the build-up to Christ’s sudden appearing. Learn the ‘semaphore’ language of the signs of the Book of Revelation, which identify God’s plan and flag up who the real enemies of God are in this world scene prior to Christ’s appearing.
Source: The Revelation of Jesus Christ by John Metcalfe (The Publishing Trust 1998 ISBN 1 870039 77 7) – various pages as detailed