lemmy.world
started on a 2vcpu/4GB instance which worked until it was like maybe about 1k registered users / 300 active users. It since scaled up to 4vcpu/16GB: https://lemmy.world/post/22070lemmy.ml
ran on 4CPU's until it was like 30k registered and 1k active: https://lemmy.ml/comment/451028.sh.itjust.works
runs on pretty big hardware, but they actually posted resource utilization graphs: https://sh.itjust.works/post/4706. I think they were maybe 100 registered users and 20-50 active at the time this was originally posted (they're one of the fastest growing instances and are much much bigger now). Seems like maybe 1G of ram usage?
These are all big instances, but they show that lemmy doesn't take THAT much hardware even for hundreds or thousands of users. I don't know exactly what lemmy scales down to for a single-user instance if that's what you're after. But requires a postgres db and 3 other processes (lemmy, lemmy-ui, and pict-rs). It seems likely that 1CPU and between 512MB and 1GB of ram would be somewhere near the minimum resource requirements. Not sure about disk-usage, I haven't seen anyone discuss it.