According to the latest report, it appears that AMD R9 Fury X can work in both 4-way Crossfire without problems as well as 8-way Crossfire under Linux, at least if you can get your hands on that many Fury X cards.
According to Tweaktown.com, which was present at AMD's Radeon Fury X unveiling event in Australia, AMD's Chief Gaming Scientist, Richard Huddy, confirmed that AMD Radeon R9 Fury X can be used in a 4-way Crossfire configuration.
Huddy actually replied that that it can be used in such configuration but also confirmed that the team have been playing with 8-way Crossfire setup under Linux. He also added that due to AiO liquid cooling solution, such configurations are quite hectic, even when the biggest PC case is used.
While we certainly doubt that there will be any significant and justifiable performance gains with such configurations, it will certainly be cool to see a 4-way or 8-way Crossfire configuration with AMD Radeon R9 Fury X.
Considering the limited availability of the Radeon R9 Fury X, it will be some time before we see some tests of such configuration as we are yet to see a single Crossfire test.