What have been the justifications by organised Christian groups on why around 500 witnesses are convincing when Legion can control thousands?

Legion can control thousands

Of demons.

500 witnesses

Humans.

they were about two thousand

Pigs, which, unlike both humans and demons, are mindless creatures, thus far more easily controllable; and, even then, the demons still could not enter them without Christ's permission.

There's no need to invoke the American Southern Baptist Convention when a billion or so Christians all over the world have equally been convinced that Jesus Christ is risen from the dead as were those 500 eye-witnesses to the risen Christ. As the risen Christ said to doubting Thomas who touched him then proclaimed him as his Lord and his God, "Because you have seen me, you have believed; blessed are those who have not seen and yet have believed" (John 20:24-31, do read the whole section for yourself.)

Billions of Christians, over nearly 2,000 years, form those blessed ones who believe in the power of the risen Christ without actually having seen him, as did those 500 eye-witnesses during Jesus' 40 days alive after his crucifixion.

Just before he returned to heaven, he told the group witnessing that miraculous event, "ALL authority in heaven and on earth has been given to me" (Matthew 28:18). Whatever power Legion, or Satan has, cannot begin to approach that level! The devil is a dead demon walking - and he knows it, because the last book in the Bible unfolds his deceptions, his desperate attempts to usurp Christ, and his final punishment - being thrown into the eternally burning lake of sulphuric fire. Read Revelation from chapter 9 to the end of chapter 20, for the devil's minions, the demons, and people who follow them, are also cast in there.

Satan's control tactics are deceptions and the fear of death. Christ is The Truth, and he has conquered death, sin and the devil, by his crucifixion and resurrection. Christians know this and willingly submit, in love, to their loving saviour. And even if Christians find themselves in a minority, to have Christ in them, the hope of glory, makes that a majority.

But nothing about Christianity will ever make sense to you until your eyes are opened by the revelation of God as to who Jesus Christ really is. You will need to humbly read the Bible and pray for God's understanding to have your eyes opened. Meantime, contemplate that excellent answer by GratefulDisciple, to discover where your thinking starts to go astray. May it get back on-track when you pick up the Bible to learn about Jesus Christ.

Both stories are important but each has a different purpose:

• the legion story (Mark 5:6-13) is to demonstrate Jesus's power over devils, even over thousands.
• the resurrection story (1 Cor 15:5-7) is to demonstrate that God indeed had raised Jesus from the dead, a cornerstone belief of Christianity, thus sufficient number of eyewitnesses are important. In this case it's not only to the private circle of the 12 apostles, but also to 500+ others who could see Jesus with their own un-aided eyes that the same Jesus who died on the cross was indeed alive.

All Christian groups believe that both stories are extremely important in different ways, thus God (through the NT writers) preserved the stories for us, so that we too can believe that:

• Jesus has conquered the devil (Legion story) so we don't have to be afraid of them anymore
• Jesus's resurrection is what we hope for ourselves after we die, as long as we are united with Jesus in baptism through faith in Him

Finally, there are three (3) misunderstandings implied in the framing of your question, which came from a category mistake when you compared the Legion's power over thousands with God's "power" over the 500 witnesses:

1. While the Legion controlled thousands, God did not control the 500 witnesses since they retained their free will. What God did was to arrange a situation where they could all (together) witnessed Jesus's being alive with their own unaided vision. We just need enough number of witnesses apart from the private circle of the 12 apostles to increase the credibility of the Apostles's testimony. For example, appearing to 500+ "AT ONE TIME" (1 Cor 15:6) serves to disprove the hallucination theory, a very important feature of the apologetics mentioned in the Lee Strobel's 1998 book The Case for Christ which has been turned into a 2017 movie about Lee Strobel's journey to faith. (When he was still an atheist, Lee Strobel was an award winning investigative legal journalist for the Chicago Tribune).
2. As explained above, the number itself is not the point of each story as long as the number is sufficiently large, because the two stories have completely different purposes and are meant to convince us in different ways. Therefore comparing the number 500 and the number 1000/2000 is missing the point (apple vs. orange comparison).
3. But if you still insist on a numbers comparison, Jesus / God still won because while the "legion" had power over thousands, please realize that Jesus had power over the "legion", so technically Jesus had power over thousands as well.