Please read the Quick Start guide for Hardware requirements:
System Requirements for Zimbra Collaboration Suite 6.0
As a rule of thumb any decent Server coming with two Multicore XEON CPUs, 8 GByte or more of RAM and at least 4-6 SAS drives - RAID10 preferred - will handle thousands of mailboxes without any problems. If you are a service provider you will probably use a multi server setup to distribute the load of MTA and Mailboxd to different machines.
Also you may want to search the forum for related discussions. The topic has been covered several times:
Hardware Requirements