I think this topic comes up every six months or so . . .
We do keep the majority of our stallions in a bachelor herd year-round. They range in age from 1 yr to 20 yrs and from 25"-34". The caveat for that is they're on 20 hilly acres so have plenty of space to exercise and hang out together. There is a 1/4 mile of common fence line to the mares too, so they do see the mares and interact through the fence. The most dominant stallion or two will claim the majority of the fenceline.
The boys pasture in 2003, I don't have any recent photos on our website.
Most of our boys have been raised together from birth or were under 2 when they came here. We do have a couple of older ones that were 9+ and although they probably would adjust fine, I chose not to put him in with the other boys, but he has at least 1 to 2 mares with him year round. My other stallion, (had WNV in August), so we've chosen to keep him close to the house. He will be babysitting our weanling colts when we separate them in November from the fillies.
You really learn alot about herd behavior watching different groups interact.
I have noticed though that two to three stallions together will fight more than a larger group will.
All the boys have buddies, even the most dominant one will have his pal to hang out with, share hay, cruise the fence line with, groom, etc.
Stallions play fight, and fight for real - most of which is screaming, pawing and strutting. It can get rougher, but usually ends within 1/2 hour maximum time. Mares on the other hand can carry a grudge and fight for days - we've lost a foal to pregnant mares fighting.
If you have stallion(s), please research independently the options available to you (including discussing this with your vet!) and don't let others dicate to you your choice. They're your horses and you are the only one that can make the best decision on their care.
This site has two articles on stallion's testosterone levels in a bachelor herd situation (the whole site has lots of articles on breeding & reproduction).
Stallion management article