Quota is unlimited and depends on first come first serve basis. USually there are 2 LB's reserved under the Lower Berth Quota in every class for the Senior Citizens. Once those two are booked, the system automatically tried to place them in other LB's (if available).
OTherwise they are allotted any random berths. (PRovided the choice for LB is not exercised during booking)
Even...
more... in the latter case, SS would be availing the discount in booking the berths.