How often should a recreational boxer spar ?

You should eventually do some sparring under supervision of a seasoned coach and ref. If you are working with a coach a good one will know when you are ready to spar and who to match you up with. I've seen too many bad coaches who push kids too soon. They pair them up with some killer who hurts them and they never come back to the gym again. You shouldn't need too much sparring if you aren't going to compete. Spar a few times so you know what's like to get hit, how to develop your reflexes and move on your feet. You will quickly learn your limitations and if you aren't very good at it you may find yourself talking your way out of confrontations (which you should be doing regardless of skill level) rather than puffing your chest out and throwing hands in a fight you could have avoided.

But sparring will get one better prepared to handle a street fight than throwing punches at air. It’s all a matter of opinion, I was in the gym for about a year before I started sparring and competing and the floor work I was doing prepared me well for the altercations I was in If anything the sparring held me back on the street because it was ingrained in me to fight clean, I never had that problem before I started sparring

Sparring and fighting in the boxing ring is nowhere near comparable to fighting in the street But sparring will get one better prepared to handle a street fight than throwing punches at air.

throwing punches at a bag or shadow boxing is nowhere near comprable to actual sparring and or fighting. Sparring and fighting in the boxing ring is nowhere near comparable to fighting in the street

I might be in the minority but I believe if you are not going to compete why spar at all If you are just in the gym to learn how to throw punches properly that’s all you need for self defense You never know how many punches you can take before it’s one too many, until it’s one punch too many, and then it’s too late the first punch you take will cause permanent damage in the long run The human brain wasn’t designed to take that kind of damage, obviously I stopped sparring when I stopped competing, I still roll to this day after 55 years of wrestling, but I’m not getting punched in the face so I’m cool throwing punches at a bag or shadow boxing is nowhere near comprable to actual sparring and or fighting.

If it was me, no more than once a month, and take months off if you get knocked down. You should be spending way more time on drills than sparring. After a few dozen sparring rounds you'll start to feel a little more comfortable in there and that's probably plenty for self defense.

How often should a recreational boxer spar to learn self defence but avoid notable head trauma? I don't think you should spar unless you're competing at someone else mentioned already. You're basically giving yourself minor/major damage that is unnecessary. You can still have someone throw punches at you and do pad work that can mimic punches coming at you so you can practice dodging without taking the impact to your head. Plenty of actual fighters who do fight professionally do this when they're not sparring. Just stick with that part of the training. What's the point of sparring for a potential fight you may never encounter in your life, only to get messed up worse from those sparring sessions?

I might be in the minority but I believe if you are not going to compete why spar at all If you are just in the gym to learn how to throw punches properly that’s all you need for self defense You never know how many punches you can take before it’s one too many, until it’s one punch too many, and then it’s too late the first punch you take will cause permanent damage in the long run The human brain wasn’t designed to take that kind of damage, obviously I stopped sparring when I stopped competing, I still roll to this day after 55 years of wrestling, but I’m not getting punched in the face so I’m cool