MME serves multiple eNodeB's. MME should allow to attach the UE's from all the eNodeB's under this MME.
For example, if eNodeB1 and eNodeB2 were registered under MME. eNodeB1 assigns x number of eNodeB UE S1AP ID's for x successful attached UE's. Also, eNodeB2 assigns y number of eNodeB UE S1AP ID's for y successful attached UE's. But MME should assign (x+y) MME UE S1AP ID's.