In a book full of 'signs/symbols' we have to interpret carefully. Who are the 144,000? They are those who have been sealed by God in contrast with those who have received the mark of the Beast. In other words they are Christians, for ALL Christians have been sealed by the Spirit. That the number is artificial is found in that the tribe of Dan is omitted (why should God ignore a whole tribe?) and Ephraim is included as Joseph. This was of course because the names of Dan and Ephraim were especially linked with idolatry, something which is firmly rejected in Revelation..
The 12 x 12 and the 144,000 are found again in Rev 21 of the New Jerusalem. What then is the significance of the 144,000. It refers to 12 patriarchs and 12 apostles (Rev 21), multiplied by a number which means a large indefinite number (1000). Its specific nature indicates that God has His people 'counted'. Its general nature indicates 'the multitude which no man can number' who are Christians through the centuries. God had them counted, man has not.
Thus the 144,000 indicate all God's true people witnessing in the world