UE mm will be maintaining TA list and TAC will be received in sib1 if I am not wrong.
So UE will check whether TAC is there in the list if not TAU request will be triggered to MME.
I think in Attach accept and TAU accept, MME provides TAList to UE.
Also there is no fix numbers of TAC in 1 TA list.
Can be from 1 to max number supported by MME design like in my case 16 but TAList is dynamic in our netwrk.
If it finds UEs do frequent TAU then that TAC gets dynamically added in the TAlist to avoid frequent TAU.