Q. I see few kernels while booting my Linux system. My question is on Linux how many kernel you can compile at the same time and how many Kernel you can load in Linux?
A. You can compile as many as kernel (or different version of same kernel with different features such as router or particular device driver) you want. You are limited by disk space only. However you can load only one kernel at a time on a physical system.
There exists a combination of hardware and software that allows you to load multiple Linux kernel using special technique called Virtualization. It is the technique of managing and presenting storage devices and resources functionally, regardless of their physical layout or location. For example XEN or Vmware software provides Virtualization facility to load multiple kernel and application on a single physical system.