What happened to Gellert Grindelwald's supporters in Bhutan? When Gellert Grindelwald was announced the next Supreme Mugwump by Anton Vogel, he had hundreds, maybe over a thousand, followers who all shot his symbol high into the air, in a temple in Bhutan. But when the duel between him and Albus Dumbledore began, we don't see them anymore. What happened to them? Why didn't they help Grindelwald fight?
They likely think Grindelwald capable of winning on his own.
Wizards don’t usually attempt to assist other wizards in duels - duels are usually fought between only two people. Presumably, Grindelwald’s followers believe him to be skilled enough to win the duel himself.
“‘Impedimenta!’ yelled Harry. His jinx hit Amycus in the chest: he gave a piglike squeal of pain, was lifted off his feet and slammed into the opposite wall, slid down it and fell out of sight behind Ron, Professor McGonagall and Lupin, each of whom was battling a separate Death Eater: beyond them, Harry saw Tonks fighting an enormous blond wizard who was sending curses flying in all directions, so that they ricocheted off the walls around them, cracking stone, shattering the nearest window –”
- Harry Potter and the Half-Blood Prince, Chapter 28 (Flight of the Prince)
They also might not have wanted to risk hitting Grindelwald.
Additionally, when multiple wizards start casting spells in an area, they run the risk of hitting people on their own side. A Death Eater accidentally killed another Death Eater that way.
“And a Death Eater’s dead, he got hit by a Killing Curse the huge blond one was firing off everywhere – Harry, if we hadn’t had your Felix potion, I think we’d all have been killed, but everything seemed to just miss us –”
- Harry Potter and the Half-Blood Prince, Chapter 29 (The Phoenix Lament)
Additionally, some may not want to publicly support him.
Also, some of Grindelwald’s supporters may not want to publicly show themselves to be supportive of Grindelwald’s cause, instead choosing to stay quiet so if he loses they might escape punishment. Lucius Malfoy did this on the Death Eaters, never showing public support for Voldemort.
“‘Lucius, my slippery friend,’ he whispered, halting before him. ‘I am told that you have not renounced the old ways, though to the world you present a respectable face. You are still ready to take the lead in a spot of Muggle-torture, I believe? Yet you never tried to find me, Lucius … your exploits at the Quidditch World Cup were fun, I daresay … but might not your energies have been better directed towards finding and aiding your master?”
- Harry Potter and the Goblet of Fire, Chapter 33 (The Death Eaters)