This really depends on what you are editing. 13mp snaps from your ancient DSLR? Yeah, no point. 52mp snaps from your digital medium-format camera? You'll probably want more than you'd think. 4x5 or 8x10 negative scans? Open the bloodgates.

(and remember that by default Lightroom does not parallelize across multiple photos within a batch job, so multiply your measurements by 4x or whatever if you're going to be launching multiple batch jobs at once to fully occupy a high-end processor.)

128 GB is overkill, but there's a solid justification for at least 32 GB or 64 GB for power-user situations.