We generally don't recommend adding more than 1000-1200 users on a given server; here's why:
1) Mailbox sizes always grow; so any benchmark you take will be out of date soon enough
2) More and more messages have attachments; and people want to view them
3) MDS is being utilized a lot and with lots of companies restricting usage to the MDS Browser its even more load on the BES
4) Your Exchange mailbox servers may have more than just Exchange on them; such as A/V scanner, Spam filters, Archiving, Agents ... each of these things adds more overhead for Exchange which hits BES harder.
5) If one of your BES dies, you have enough theoretical capacity to move the rest of the users over if you can't get a server up and running quickly
With all that said, if you're ESX servers are 8-way multi-core processors with 64gig of ram attached to ridiculously fast arrays, then you should seriously consider virtualization for at least the following:
1) RIM Supports it
2) If you have to fail over, failing over a VM _can_ be cake compared to failing over hardware
You certainly don't need that much HW for a VM either; but just giving an example of what some places are doing ... virtual EVERYTHING.