It depends on the distance across the baseboard front to back and whether it is a solid board across that width. If it is about 40cm or less and solid then I don't think that any cross battens are needed. That is the case with my storage sidings. If it is greater, then I would suggest about every 40cm would do. That is what I have on my layout and the 9mm plywood seems fine.
Well it largely depends on your surface board, mine are 12mm MDF so I have large boards about 1 metre across so I have intermediate bars of pine at about I metre square, but MDF is nioce to use but can sag a biot, plywood is variable but a bit stiffer so my upper level has more plkywood. Father used 18mm chipboard which was solid so it's down to what is on top really but all these thinner boards sag unless supported and this leads to a sort of hill where they join up if you use 9mm board then this needs more support as my narrow gauge is 9mm MDF I added another layer and the frames it sits on are metal so stiff enough to avoid sagging or ditortion.
Pragmatically, the minimum is the distance needed so you can fit your drill between them to drill new holes... All material sages, but a "Stiffening strip"can easily be placed under critical track-work as long as the basic structure is solidly built.
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