There are some other posts with some explanations. But my rules of thumb are:
Make the texts in vectors.
Make the background at a pixel size from the longest side between 6,000 and 12,000 pixels.
This is because the bigger the printed image the further away you will see it the less ppi resolution you need.
My numbers are chosen as some nice round numbers. A 6000px size image is:
300ppi in a poster of half a meter.
150ppi in a poster 1 mt tall.
75ppi on a 2 mt tall print. etc.
You can push this with double the size for a 300ppi file at 1 mt. But I only recommend that for some art prints.
With your image, if you make it with 6000 x 6000 px you will have a resolution of 25ppi which is great. (6000/20/12=25)
Is a pixel of 1mm and you are probably seeing this image at 10 feet or something. You can barely see a pixel on a photo at that distance.
With this numbers you can make an artboard of a 1/10 the scale at 250ppi. Artboard size in Ilustrator, for example, is around 270 inches.
Remember to keep the texts on vectors as possible. If you need to send a flat raster image, you can export this image with the vectors at double the resolution, to have the texts at 50ppi on the final image.
Personally, I would NOT make the paneling. If the final panels does not fit each other people can blame you.
You need to take into account overlapping, roll with, roll print area, etc.
And there is a chance the print house has some automatic process to do that.
A couple of things you need to ask is if they use an RGB file, if you can include vectors with a pantone defined color, or they need CMYK values, and the desired color profile.