Hi,
SR periodicity is used for delay between two SR transmission but it doesn't mean only one UE can transmit in that SF, in single SF based on sr-PUCCH-resource index many UE can transmit the SR. SR doesn't carry any data , it is just notification to ENB saying need some grant for BSR, If you feel many UE (like 96 UE) is about to send SR then ENB can reduce it by giving some periodicity grant to UE. So even at higher no of UE ENB can handle by giving random grant for the BSR.