Hello, Peter:  I sense a bit of terminology confusion in your message.  You write that you want to know how much memory a machine has, then mention an amount of 1 TB.  The amount of memory, more precisely, random access memory, RAM, in your XPS 8900 is probably 16 GB, which is well short of the 1,024 GB that can be referred to as 1 TB. The important characteristic of this memory, which is located on the main circuit board of the 8900, is that it vanishes when you turn off the power to your machine.  This memory amount could be increased to 32 GB, or even 64 GB, by adding to or replacing memory modules on main circuit board.  But that won't solve your problem.

It appears you want more permanent storage in your machine, which probably arrived to you with a 1 TB, or a 1,024 GB hard drive, is the location of your machine's longer-term storage. This drive is located in an aluminum cage inside your 8900 machine, in a drive bay near one or two similar, and likely empty, drive bays.  The easiest way to add permanent storage to your machine is to install another hard drive in one of the empty bays, then connect data and power cables between it and the main circuit board to enable it to serve you.  This new hard drive might be larger than the existing one, say 6 GB, or even more, and it could provide you with plenty of storage space in your Dell XPS 8900.

On the other hand, if you have your heart set on buying an entire new computer, such as a new XPS 8950, depending on the specifications you order, it could  have similar, or even more RAM installed, and a 1 TB hard drive, too.  It will also likely arrive with a newer kind of storage device called a solid state drive, SSD, which is an electronic, not a mechanical, device, that provides permanent storage. Often the capacity of these devices is 1 or 2 TB, and the machine's operating system is installed on them, so the entire machine operates much faster.  SSDs come in different forms, some in plug-in slots, and others residing in drive bays; there are pluses and minuses to each form.  You can view the price differences on the Dell site order details pages.

Oh, by the way, the answer to your question about maximum storage capacity depends on knowing that external boxes of hard drives can be attached to a desktop machine.  With this thought in mind, there really is not a practical limit, except money, space, and electricity costs, to how much storage can be attached.  One or two terabytes might be inside the desktop, and dozens of them outside it.