The TRS-80 Model I came with 4KB, 16KB, or 48KB of RAM. The TRS-80 Model III came with either 4KB or 48KB of RAM.I remember when they didn't even mention the RAM. TRS80 Model III -- yes, I am that old!
I guess I am even older. I had the TRS-80 Model I. I started with 4KB and upgraded to 48KB.
Only 48KB of RAM max could be used because the lower 16KB was reserved. It was used for memory mapping to hardware such as BASIC ROM, keyboard, and video.