[Keeper of Dol Guldur]

Say after the Battle of Five Armies Beorn settled down with a forest woman, and had ten or so kids (in the course of the next ten years). Twenty years after his last son, they've grown up, ranging from 20 to 30 years of age. They have kids. 20 to 30 years later (we've approached seventy years after the Hobbit), their kids are old enough to fight in the War of the Ring. We now have 10 first generation Beornings (obviously the eldest being Grimbeorn the Old), and 100 grandchildren to Beorn. That's obviously an entire 'race' of Beornings, at least that's a lot of men. Also, with the forestmen living nearby, it's not reasonable to think that in the dangerous times, they rallied under the greatest leader in the area-Grimbeorn. Therefore, beside the true 'Beornings' there were probably upwards around 2,000 or more good fighting men in the region between Mirkwood and Anduin. Not to mention their children and wives, and elders. So yes, in 80 years it's possible that not only there are quite a few Beornings, but enough people following them to make a decent army.

I've theorized before that Radagast the Brown probably served a role much like Gandalf did for the characters of the Lord of the Rings, and acted as the counselor and mover of the Beornings, and the men of Dale and dwarves of Erebor, and also the Elves of Mirkwood.

So they had some numbers and organization, I assume. 80 years is a long time, even for long-lived offspring of a skin-changing sorceror, but if he did have more than one son, it wouldn't be very good for the forces of Dol Goldur to have ten or so were-bears charging them down with a hoard of angry militant forestment behind them now would it?