I have not found any proper reference, so I have concluded that up to 4 different RRC signalling messages can be send to UE from eNodeB using values 0 to 3.
For example: RRC connection reconfiguration sends to UE with rrc transaction identifier 0 and at the same time Mobility from E-UTRA sends with rrc transaction identifier 1 to UE.
But I don't have confidence on the my answer given above. I am waiting for others to respond.