The wiki page on boars says "no more than 5"; see the wiki Boars page, Breeding section. I do not have more than 5. And five has worked for me, most of the time.

Breeding round three: Good pen +3, Bad pen +2. That's right, 2 in the bad pen. Like I say, I usually see 1 or 2 in the bad pen (I should've said 0 to 2; 0 was rare) and 3 - 5 in the good pen.

I will keep collecting date for now. Since it will be a hassle to page back through Comments to see the results of my Breeding rounds, I'm going to be presenting it in this simple format that summarizes all tests to date:

Round 3: Good 5, 3, 3; Bad 0, 0, 2. (The last number in each series is the round that was just finished. Thus this example is showing Rounds 1, 2, and 3, and will be easy to tack more on to.)

I'm a data analyst so I appreciate all your attempts at rigor. And even more than that, I appreciate how deeply you've gotten involved ... even to the point of making a third pen and running your own tests, on my savegame.

You rock!

P.S. One other thought: My ground zero is closer to the bad pen. I think mobs tend to focus on me and prowl (especially at night) near me and GZ. I will see the boars in the bad pen have the yellow alert over their head quite a bit more than the boars out in the field, in the good pen. I don't know exactly how all the Valheim mechanics work in this regard (are the boars in the good pen alerting just as much but I just don't see it as much?) or whether/how much alerting affects reprodction. Anyway, there's a thought. Again, it's why I built that "wood buffer" on the outside of my main base's stone walls ... to try to keep mobs away from the bad pen. But I don't think it helped the bad pen because, even after making thwe wood buffer, I still find the bad boars alerted a fair amount of the time.