The Hall of Famer explained his theory on The Bubba Army radio show this week ... after he was asked how many serious head injuries he had while playing QB in the NFL.

Favre said if he had gotten that question a few years ago -- he would've said just three. But, after digesting recent studies on concussions, the 52-year-old believes the number is now actually in the four-figure range.

"Concussions happen all the time," Favre said. "You get tackled and your head hits the turf, you see flashes of light or ringing in your ears but you're able to play -- that's a concussion."

"So, based on that, [I've suffered] thousands," he continued. "Had to be, because every time my head hit the turf, there was ringing or stars going, flash bulbs ... but I was still able to play."

Favre was obviously never formally diagnosed with that many -- but he certainly did hit the turf a lot in his 20-year career. In his stints with the Falcons, Packers, NY Jets and Vikings, he was sacked 525 times just in regular season games alone.

One of Favre's scariest head injuries came in a 2004 game against the Giants ... when the Packers star threw a touchdown pass to Javon Walker after hitting his head -- something he didn't remember doing, according to then-Giants QB Kurt Warner.

"That's what's kind of frightening about the concussion thing -- it's the ones that seem minor that do the damage 'cause you're able to play and keep going," Favre said.