im a newbie of upgrading my audio system

currently, i have stock bose system with stock HU

Im thinking to buy the JL audio 2 subs and 1 amplifier

my question is:
1.whats the difference b/t mono channel amp and 4/5/multi channel amp?
2.Why do some people buy 2 amps?
3.what cables do i need to hook up an amp with 2 subs to my stock HU?
3. any suggestions on what model of sub and amp i should get, my budget is around 600-800 for subs and amp, thanks!

thanks for reading and any replies will be greatly appretiated!