How much head trauma does it take to get t.B.I.?

Just very little. Tbi is a broad expression for a trauma to the skull/brain. A tbi can vary from a small bump of the head causing a brain injury to severe accidents causing skull fractures, brain swelling and massive damage. I would consider any even small trauma to the head a tbi.
Depends. Losing consciousness, or even just feeling confused can indicate a concussion, which is the minor form of traumatic brain injury. Doesn't take much trauma, being hit with an object or falling down and hitting the head are sufficient.