I design the stem to satisfy shear demand without requiring shear reinforcement, then design the flexural reinforcement. If the resulting reinforcing ratio is excessive, increase wall thickness thereby is required. The front face bars are usually treated as good practice for temperature and shrinkage control. Some of the designers provide single layer of steel only. Depending on the practice of your company.

Agree with retired13. Start by checking the shear at the stem base and size the thickness based on an unreinforced shear check. That will get you in the ball park. Depending on your code, over 9" thick will start to require you to go to a double layer of reinforcing. For private residential jobs, a single layer of reinforcing will probably suffice for shorter walls. On transportation projects, I would never consider going with a single layer of reinforcing, as 12" thick is typically my bare minimum stem thickness even for short walls.

When scale would indicate a double layer, it also is more constructable, even when a single layer is possible. The bars can be tied or flown in with more stability before closing up the forms. Otherwise you're looking to build one side of the forms first or using temporary bracing.

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just call me Lo.

If you are providing compression steel, keep in mind that it must be tied as per the code requirements in order to be considered. I would increase the tension side reinforcing until you meet your demand, and then the other reinforcing doesn't need to be tied.

Or as indicated by others, increase the wall thickness until compression reinforcement is no longer required.

I have never even considered using compression steel in a retaining wall. But then, I haven't used compression steel often in anything but columns.

Just a side question as not to hijack the thread,
how is the horizontal shear transferred from the wall to the base?
As the foundation and wall are generally cast separately, thus "cracked".
Is it not just the tension reinforcing going through the interface that
transfers the shear?

I guess this rarely governs.

Yeah, it's just shear across a cold construction joint, but the provided flexural steel is practically always enough to develop shear friction across that joint.

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just call me Lo.

On heavily loaded wall/base junctions I'll provide a shear key so there's a positive connection.

Thanks a lot all for the replies, well appreciated.