Basically as some other have said. A ‘heard’ to me is a very large population. I hardly ever hear the term used except on the forum or out west. I prefer groups At least 2 up to around 6 in a large paddock. Our mare field will have as many as 15+.
But it all depends on the horse/s. Just like people not all like and or play well together. All of our horses are introduced before being turned out together. The majority of our stalls have wall with bar partitions tops. A new horse will be stalled between his/hers new paddock mates and I monitor how they interact. Go from there.
We have some small paddocks that share a fence line with group paddocks. The new horse will be turned out in there so they can sort things out over the fence. We monitor how they interact and go from there.
I have found mares by and large are more problematic then geldings. Colts by the time they are 3 year olds and some late 2 year olds can get very rough with each other.
Depending on the size of a given years foal crop we may or may not wean by gender until they are early yearlings.
I would not turn out colts, entire horses together. Not even a single colt with a geldings. Stallion instincts takes over.
Regardless of gender of older horses ‘pecking order’ always take a few days to sort out. Especially if fed in the paddock. It’s interesting how it changes over time. IME geldings sort it and it remains pretty much the same. Mares not so much.
IMO and experience if properly introduced and grouped according to personality/kinship and kept that way the possibility of injury is slim to none. At least not IME and we have a fairly large population with horses coming and going at times. Minor nicks and scrapes are a given at times.
As to monetary value that’s not much of a concern when done correctly. We only work with TBs a lot of which would be valued much higher than the average farm owner’s horses. Consider most of the world’s most valuable horses are raised in large groups in Kentucky I’m not the only one that feels this way.
If turned out 24-7 by and large it is important not to let them, any two get heard bound. IMO one has to take the time and effort to stall them separately from time to time. But this depends on the horse/s
The only horse we lost to a paddock injury was a yearling colt turned out by himself. Perfectly safe paddock but somehow he broke his shoulder. A mystery to this day.
This is how I feel about it. To each their own.