Can anybody tell me why I have 12 wires in light box?

3 supply, 3 supply to next light, 3 for switch, and possibly 3 for a duplicate fitting controlled by the switch.

As other comments said, just use the connections you need and leave the others alone.

With old wiring, it's hard to say what's what. In old houses it was common to feed the light with power, and feed the power to the switch. The other wires could be anything from an outlet on the same circuit, a light on the same switch, a light on a different switch, or a completely different circuit that just uses the box as a junction.

I'd agree with the common advice of just ignoring the extra, and hooking it up like it was originally.

If you're really curious, you'd have to get out voltage testers and turn off circuits and switches, disconnect wires and walk around the house to see what does/doesn't work to know what's going on. Unless you're doing a larger re-wiring project this is unlikely to be of any value.