ASUS Prime B760M-A D4 Intel® B760 (LGA 1700)(13th and 12th Gen) mATX Motherboard:
https://www.amazon.com/gp/product/B0BR8S9KTT:
Intel Core i9-13900 Desktop Processor - 24 cores (8 P-cores + 16 E-cores) - 36MB Cache, up to 5.6 GHz - Box
https://www.newegg.com/intel-core-i9-13900-core-i9-13th-gen/p/N82E16819118425
2x Crucial RAM 64GB Kit (2x32GB) DDR4 3200MHz CL22 (128GB total):
Yes, it's a high-end build, but I rackmount everything and I only have so much space in my rack -- so, I wanted to be sure I had plenty of horsepower. I reused a very nice Silverstone 2U case with room for 4x 3.5" drives, and 2x 2.5" drives -- plus the motherboard has 2x NVMe slots.
Obviously, you don't need to duplicate the above in any way. Keep in mind though, when creating VMs and CTs each is going to be assigned a max number of cores it can use and RAM it can consume. It's OK to "over-provision" on cores, though it'll slow things down if all of your active instances demand is high at the same time. RAM though, should not be over-provisioned.
If you can find something in a form factor you like with say 8 cores, and 32GB of RAM you'd be in pretty good shape I would think with room to "grow". If you weren't going to allow for any Windows VMs, and focused on using strictly Linux LXCs, you could probably get by with 4-6 cores and 16GB of RAM.
Proxmox is a "virtualize everything" environment -- you don't want to install any packages directly on the host (with the exception of Tailscale). LXC containers are best at sharing resources, and can run Docker on top. VMs are good for anything with a desktop environment, and some specialized stuff.