Is there available Audacity 64 bit so we can use many 64 bit VST plugin?
From the website (Audacity ® | Downloads)
Please note that for Windows and Mac, Audacity is only available as a 32-bit application that may be used on 32-bit or 64-bit systems. 64-bit builds will be available in due course.
64 bit will be available “in due course”. This was 2 years ago. When will there be a 64 bit version?
Audacity is now 64-bit on macOS.
On Linux it may be built as 32-bit or 64-bit but we only provide the source code.
Audacity 2.4.2 release build is 32-bit, but I believe that it can be built (on 64-bit Windows) as 64-bit from the source code.
The next Audacity release for Windows ‘may’ be 64-bit, but that will be a big disappointment for any 32-bit Windows users, and may be an inconvenience for Windows users that have 32-bit VST plug-ins. I’m not aware on any final decision on that as yet.
Wouldn’t 32-bit VST plug-ins be backwards compatible, still work on the 64-bit version?
The general rule is that 32-bit VSTs are for 32-bit hosts, and 64-bit VSTs are for 64-bit hosts.
I would hope that when Audacity for Windows becomes 64-bit, the developers will be able to make a “VST bridge” to allow 32-bit VSTs to still work, though I don’t know what technical difficulties there are to doing that.
Getting on towards 2 years later:
Audacity is now 64-bit for Windows (a 32-bit build is also available if you search for it on the download page).
Unfortunately it seems that Audacity getting a 32-bit/64-bit bridge for VST plug-ins will not happen any time soon, if ever. So when upgrading from 32-bit Audacity to 64-bit Audacity, it will be necessary to update any 32-bit plug-ins to 64-bit versions (or if you really need to use a 32-bit plug-in that is not available as 64-bit, install the 32-bit version of Audacity).