Probably depends on what direction you think you will go in long term. Companies tend to gain knowledge and expertise over time so it less common to to shift hosting models (except early on).

At just one rack, if you want to own hardware you would usually just rent a single cabinet (Can be pretty cheap depending on location).

The number of servers in a rack is variable depending on the power. We got 208v / 30A , so we can pretty much fill up a rack with redundant power.

In our main facility, we currently have 4 racks in use which are not full yet and a 5th already provisioned for growth.

I would disagree. With modern processors and memory you can cram a LOT into one cabinet. We run with Dell C8000's and R620/R720's and they are 'wow' level fast. Given that you can cram dual processors and 256GB of ram * 8 into 4U with the new C8000's you basically can have around 2TB of memory and 16 physical cores in 4U... that's a lot of hardware.

Addendum: I should say though that be aware the C8000 servers are hungry beasts for power. I believe they are nearly 9A at full draw.

I worked at a place where the hardware in one rack easily exceeded 150k. We started saving money vs. a cloud solution in about 6 months. You can pack a lot of firepower in a virtualized environment.