T480 can take 64 GB (2x 32 GB); no idea if more is possible. I imagine newer models could but I struggle to remember seeing 64 GB SO-DIMMs... P15 can fit four sticks so that should be possible, but them things have beefy CPUs, are rather large, and also have Nvidia graphics so dunno how low-power you can make those.
you're kinda outside of the intersection of cheap and still capable with that spec. do make a write-up if you succeed, that sounds interesting.
skip the T470, T480 with 8xxxu cpu is the lowest you should go; the hardware is practically identical (and interchangeable!) but the CPU is a huge difference. also if you find them for cheap, there's T490 (refresh), T495 (AMD Ryzen), and T14 (newer variants of the T4xx series with Intel and AMD CPUs).
the 12" version would be the X280, again single-channel RAM only. in the 12" space you also have Dell Latitude 7290/7200 (just the latitude series, no inspirons and friends) as well as HP Elitebook 820 (and 830) with 8xxx and newer CPUs. Elitebooks and Latitudes are Thinkpad T-series equivalents with similar build quality and features.