Hi ganesh,
At ENB side in one PRACH SF it can succesfully detect only one preamble. Because PRACH resource is 6 RB which is common to all UE, then If all UE tries to send PRACH to ENB then it will collide in RF and will be discarded at ENB. Based on UL delay some UE's preamble reach ENB then ENB will process it. I hope you know CRM procedure, this will let other(failed UE) UE to do rach again.