how much clearance does a US Stove Company model 1269E need to have from cement board walls and floor
The installation manual requires 14” clear behind the stove to a combustible wall and 23” on each side. (It can be less if it’s non-combustible.)

Here’s the installation instructions with installation details:

https://www.usstove.com/wp-content/uploads/sites/2/2018/10/1269e-use-and-care-manual-1.pdf

Also, they indicate you should check with local codes and it should not be installed in a trailer.