You have 10 GB of RAM, and you load multiples instances of a program of size 1 GB( let’s say more than 10 instances), then what will happen, how the OS handle this situation and up to what extends?..........plz xplain

Do some research on virtual memory and paging. The answer is that active pages will use RAM while inactive pages will be moved to hard disk space until needed. Provided there's sufficient hard disk space to hold the inactive pages, the OS will behave as if there's infinite memory (at the cost of performance in moving pages around).