Open source does run on 64-bit. it is built for RHEL6, but see
Bug 46506 – RHEL6 Server support
I would not suggest using RHEL6 with Zimbra yet, as its very broken on the RedHat side of things.
There is no user limit on the open source edition. You can add as many users and domains as you would like, and add additional servers to expand horizontally.