When Jesus rose from dead many dead rose also. There could be many people in heaven already.
Bingo in my book showing that there could also be more than a single taking up or rapture. I will make one observation of those many that rose from the grave in Matthew 27:52 at Christ's resurrection; they were not the church but Old testament saints, and then many rose, not all.
You mentioned that there could be many in heaven already, and that is certainly true because when Jesus rose He first descended to paradise and took them captive. Paradise is now in heaven also, but they too are not the church. As the tribulation progresses there will be those slain for the word of God asking for vengeance from under the altar; another group not of the church.
We can count those of the church mentioned in Revelation Chapters Two and Three. Some are promised crowns, others have a palm leaf, a white stone. And then some are promised to reign with Christ, while others will be servants in the temple of God (Revelation 7:15). Some of the church are promised to be kept from the great temptation to come upon all the earth, while others are said to enter it.
Jari, are you familiar with the Ephods and the breastplate of judgment Abraham wore? Due to their walk with God, certain tribes were demoted as it were to different ranks when worn on the breastplate. A similar judgment is occurring as Jesus walks among the churches or candlesticks today. In Revelation 2:5 Remember therefore from whence thou art fallen, and repent, and do the first works; or else I will come unto thee quickly, and will remove thy candlestick out of his place, except thou repent.
The church in Thyatira is told to repent or be cast into great tribulation in Revelation 2:22. Dosesn't that suggest there's the faintest chance that if they do repent they will be spared that tribulation?
Note one last thing if you're not tired by this time, there are seven conditions of the church described in Revelation Chapters Two and Three; there is still but one church or body of Christ regardless of their spiritual state. Two are commended and have crowns, and the remaining five are told to repent. Judgment begins at the house of God, and this is it (1 Peter 4:17). There are two groups with Jesus when He takes His throne; Philadelphia and Smyrna are the only ones to have crowns, and Smyrna represent the dead in Christ who will rise first, and Philadelphia them that will be caught up together with them to meet the Lord in the air.
BTW. Notice in Revelation 4:4 those twenty-four elders wearing crowns.
My thoughts and blessings to you in Jesus' name.