If you want to get an impression about the number of users you can support with a Horde installation you can look at the
There is also a minimalistic overview of potential hardware requirements available. Of course this depends heavily on the type of infrastructure you use as the backend for Horde but it may provide you with a rough idea. See
We also keep a list of distributions and resellers that offer a repackaged Horde that you might prefer over the PEAR based installation method supported by the Horde developers:
In case you would rather like to avoid installing Horde yourself you might have a look at the