If u will see the Diameter protocol message in whcih MME asks to HSS for authentication vector u can see that MME has an option about how many vectors he want.
Requested- EUTRAN-Authentication-Info ::=
[ Immediate-Response-Preferred ]
[ Re-synchronization-Info ]
So if he will ask for at a time then each time when ENB / MME decides to change the security context no need to ask HSS (Where each time he has to to calculation for generating the KASME).
If MME asked more than one AV in the message so that later when he want to changes then he can change the KSIASME so that the key KAME can be changed and applied.