I dont really understand this question, you don't ever have to use a loopback ever, however having one at least will simplify many many things like network management, routr ids for routing protocols etc.
Since your question mentions OSPF you can rest assured that you do not need more than one for sure.
I mean if you dont understand why you need them dont use them, there are cases where multiple loopbacks are good, for example NAT, Multiple tunnels etc
