The 144,000 are all of God's people, its God's will that all be saved. See the symbolic meaning of numbers in the Bible.
Hopefully people will see the consequences of their disobedience, repent and make amends. It's very likely that all of God's people will be praying earnestly, during the time of the seals in chapter 6. First they will be praying for God's will to be done, similar to Jesus in Matthew 26:39, Mark 14:36, and Luke 22:42. Next they will be praying that God will intercede and have mercy--so that all will not be destroyed.
Hopefully God will see that people are repenting, will hear the prayers of His people and intercede--at least delaying the destruction that is about to befall all humankind on earth! God delayed punishment many times during the time of the judges and kings in the Old Testament. Hopefully he will delay punishment again. Let's pray earnestly!!
This passage comes right after passages about the six seals in chapter 6. Most likely at some time either before or during the time of the four horsemen of the apocalypse, God will reveal plans leading up to the construction of the New Jerusalem in chapter 21. The New Jerusalem is provided by God, but God often works through people--similar to the way he provided the Ark to Noah in Genesis chapters 6-9. Like so many passages in the Bible, this one may have more than one meaning. At some critical point, the number God has called to be involved in the building of the New Jerusalem will number 144,000, but most likely only historians will be able to tell when there were 144,000. Its very likely that the people God has called will not know, even of the existence of, most of the others at the time there were 144,000 -- they may not even know they are included themselves, see: Matthew 25:37-40.
It's very likely that God's plan is to use the seals to spur the people He has called into action! Why build a boat when there is no likelihood of a flood? Why build a New Jerusalem until spurred into action by God?