There's no specific limit but remember that DiRT is just using Microsoft's ExMerge - this is why I stress so hard in the help file again and again that it is NOT intended as a replacement for a dedicated Exchange aware backup. ExMerge is slow and it takes a lot of space (it does not use single instance messaging). That said, you can backup as many users as you have time and space to deal with. For one time migrations and such it's ideal. For regular backups (i.e. to save a few bucks on a backup system) it is not ideal.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.