@Kuchbhi In Retail its lottery mostly, only in HNI its proportionate basis, higher amount you invest better chances of allotment.
Process of applying is same, since the amount will be lesser side in some SME's, operators can play around, you can be stuck with multiple UC or LC, you need to be carefull.
GMP can change fast, as other said. if its high, subscription also would be very high.
You can start with one or two retail applications and try out your luck, since it will be mostly over subscribed. Other option if you have any linked FD, you can use that amount to apply and try out.