How many gigabytes of RAM does my Pi need?

At present, unless you make manual alterations to /boot/config.txt to provide the GPU with more dedicated VRAM, more than 1 GiB of RAM will confer no useful benefit if you intend to run only TLXOS on your Pi.

There is no "gpu_mem_2048" directive, so by default our memory split is 256 MiB VRAM / 768 MiB main memory (gpu_mem_1024=256) for anything with at least GiB of RAM. According to Pi documentation, the maximum possible value is 944, but allocating any more than 512 MiB of VRAM will not serve any useful purpose.  This setting is in any case only meaningful for the Pi 3 and earlier, since the Pi 4 uses the kernel's contiguous memory allocator for VRAM, not the dispmanx firmware.

It's possible that - in future, at least - TLXOS might conceivably be able to use 2 GiB of RAM for some useful purpose if you are running dual displays on a Pi 4 at greater than 1080p resolution.  However, I cannot envisage any scenario in which TLXOS would benefit from 4 or 8 GiB of RAM (increased use of bufcache might improve a dedicated PXE server's performance a bit, but not all that much), so you are just wasting money by buying those models.