What do you base this on? The stacks and kernel bookkeeping shouldn't use nearly this much at least on linux. Keep in mind that thread stacks have are lazily allocated virtual memory so won't use as much physical memory as the thread stack size setting shows.

If these threads are handling TCP connections and L7 protocol processing on top, you're going to have nontrivial both kernel and userspace memory usage per connection too that may dwarf the thread overhead.

Here's a linux kernel dev (Ingo Molnar) benchmarking Linux in 2002 and starting just shy of 400k threads in 4 GB: https://lkml.iu.edu/hypermail/linux/kernel/0209.2/1153.html - though on a 32 bit systems lots of objects things are 50% the size compared to current 64 bit. But still gives you a ballpark.