I have OS X Yosemite 10.10.4.  There are two bundles of GHC (from ghcformacosx.github.io): 7.10.1 and 7.10.2.  The latter always shows me one core when running `getNumCapabilities` action:

GHC 7.10.2
> stack ghci +RTS -N8
Using resolver: lts-3.0 from global config file: /Users/arthurfayzrakhmanov/.stack/global/stack.yaml
Configuring GHCi with the following packages: 
GHCi, version 7.10.2: http://www.haskell.org/ghc/  :? for help
Prelude> :m +Control.Concurrent
Prelude Control.Concurrent> getNumCapabilities 
1


GHC 7.10.1
> /Applications/ghc-7.10.1.app/Contents/bin/ghci +RTS -N8
GHCi, version 7.10.1: http://www.haskell.org/ghc/  :? for help
Prelude> :m +Control.Concurrent
Prelude Control.Concurrent> getNumCapabilities
8

It looks like a 7.10.2 bug, can some one confirm that?