Resumable render for Cycles The Resumable Render for Cycle allows you to add some Samples steps when rendering your image from the command line (as far as I understand)

"This feature is also known by the name Samples Offset, which allows
artists to render animation with given amount of samples N, but then
render more samples, starting from N and ending with M (where M > N)
and merge renders together as if they rendered exactly M samples.

Surely such effect could be achieved by changing Seed variable, but
that has possible issues with correlation artifacts and requiring to
manually deal with per render layer samples and such."

https://developer.blender.org/rBf8b9f4e9bbc10675de9bbc3088f2841381e23f78

This new feature is in the master but not on the release yet afaik,

however I cannot find out how to use it ;

the links says:

"Here's a command how to run Blender with the new feature enabled:

blender -- --cycles-resumable-num-chunks 24 --cycles-resumable-current-chunk 2

This command will divide samples range in 24 parts and render
range #2 (chunk number is 1-based)."

at the moment I am trying: 
blender-b test_render_resumable.blend -f 0 -- --cycles-resumable-num-chunks 24 --cycles-resumable-current-chunk 2

then 3, 4, etc.

I get an image I backup everytime, but don't know how to merge all of them, and not sure that's the correct way to do.

Does anyone know how to use it?
Assuming that the actual renders are rendered correctly, you can try something like this to merge the separate renders from within Blender: Stacking cycles renders in the compositor?