Audacity is a 32-bit application. There is no 64-bit version.
In practice this does not make much difference except for a couple caveats (which are documented in the user manual):
* Some file paths are affected. For example the default installation directory is :\Program Files (x86)\audacity\ (see also: http://manual.audacityteam.org/man/inst ... ndows.html
* VST plug-ins must be 32-bit versions. 64-bit plug-ins are not supported.
Despite being a 32-bit application, Audacity does still use 64-bit data types in certain places. For example the sample count is handled as a 64-bit data type so as to allow > 2^32 samples.