Apache uses multiple threads to provide each request with it’s own thread of execution. This is necessary to avoid blocking when using synchronous I/O.
Nginx uses only asynchronous I/O, which makes blocking a non-issue. The only reason nginx uses multiple processes, is to make full use of multi-core, multi-CPU and hyper-threading systems. Even with SMP support, the kernel cannot schedule a single thread of execution over multiple CPUs. It requires at least one process or thread per logical CPU.
So the difference is, nginx requires only enough worker processes to get the full benefit of SMP, whereas Apache’s architecture necessitates creating a new thread (each with it’s own stack of around ~8MB) per request. Obviously, at high concurrency, Apache will use much more memory and suffer greater overhead from maintaining large numbers of threads.