He prefers 89, at least up to his time with Severstal. With Severstal, he wore 89. With Russia's National Teams, he usually wears 19. I'm pretty sure only numbers 1-30 are available for players with the Russian National Team. A Russian poster can maybe confirm that. With SKA, he wore 88. I have no clue why. There was no one on SKA who wore 89. He might now prefer 88.
The most famous player to wear 89 in the NHL was Alex Mogilny. This is just an assumption, but I think that would be where the inspiration to wear 89 came from. It certainly would make a lot of sense.
For potential numbers Buch might wear with the Rangers, he'd be the first player to wear 89. 19 isn't currently available, Jesper Fast wears it. 88 has been worn twice, Ken Hodge was first and most recently Eric Lindros.