Every process under Linux is dynamically allocated a struct task_struct structure. The maximum number of processes which can be created on Linux is limited only by the amount of physical memory present, and is equal to (see kernel/fork.c:fork_init()):

/*
* The default maximum number of threads is set to a safe
* value: the thread structures can take up at most half
* of memory.
*/
max_threads = mempages / (THREAD_SIZE/PAGE_SIZE) / 2;

which, on IA32 architecture, basically means num_physpages/4. As an example, on a 512M machine, you can create 32k threads. This is a considerable improvement over the 4k-epsilon limit for older (2.2 and earlier) kernels. 

Max_threads = mempages / (THREAD_SIZE/PAGE_SIZE) / 8

In from beginning for the i386, THREAD_SIZE=2*PAGE_SIZE, PAGE_SIZE=2^12 (4KB), mempages= /PAGE_SIZE size of physical memory, 256M of memory machines mempages=256*2^20/2^12=256*2^8 when the largest number of threads 4096.

Dành cho kernel 2.4 , Kernel 2.6 mình không biết, chắc tìm tài liệu về Kernel mới khám phá được nó .  

mempages= /PAGE_SIZE size of physical memory 

256M of memory machines mempages=256*2^20/2^12=256*2^8 when the largest number of threads 4096.