My experience is for a lab that I was involved in setting up - across the entire company, I'm sure we bought a lot more and of different configurations.How many is a lot?
What model poweredge? How many drives do they have?
For this lab, our initial purchase was for 70 units. Over the next 3 years, it grew to about 300.
The 1U units we initially purchased were R200's. Then when Dell stopped making them, R210s, R210 IIs, then R220's. These units have two 3.5" drive bays. The initial purchases came equipped with two 160GB drives. We used Linux LVM to format them as a single volume spanning both drives. Later purchases had a single drive, since we have no need for RAID.
Originally, each of these PCs hosted one instance of our application. We'd network them together in various topologies for our development and testing work. Later on, we started using VMs to get better utilization of the hardware. When we told Dell about that, their people suggested an alternate configuration, based on products they're selling for cloud computing applications. These are larger servers (2U, I believe) with two processors of 8 cores each, lots more RAM and Ethernet ports (I don't remember the model number, so I can't post that right now.) They boot a Linux-based hypervisor from an SD card and can run dozens of VMs (which are stored on a hard drive.) We now have 30-50 of these and are starting to replace the R2xx units with them, because they can run many more VMs per rack-slot.
In all cases, they were ordered by our technical people meeting (usually over conference call) with a Dell sales rep. We negotiated a configuration, got a price quote for a certain quantity, sent it to our management for approval, and then purchased them.
Our group isn't responsible for specifying the laptops we use on our desks (various models of HP mobile workstations), so I can't say how they are procured, but I would assume it's through a similar process of negotiating a contract. And I'm sure IBM and Apple will do something similar when they negotiate their contract.