Same sex/mixed sex groups - there's just as much chance of injury in one as in the other. I lost a 2 year old filly following a kick in the field and she was turned out with two other mares.
I keep my 6 horses at home - 3 geldings and 3 mares and they are turned out in pairs of pretty much any combination. It depends more on their condition than gender.
Mine's out on her own - she's a pain with others as very dominant. Mind you we even had to fence her off away from the geldings on one side so she couldn't annoy them after she cut her leg lashing out at them! The mares on the other side had the good sense to ignore her.
Now she's in a field split with electric fence between her and another mare (also a bossy cow!) They say a quick hello to each other over the electric and then ignore each other.
The others are in small groups, even the top dressage horses on the yard and seem to get on well.
I voted "same sex upto 5" and "single with horses around", as currently my mare is on her own and perfectly happy with her buddies around her, but usually she is out with one other mare, but that is on box rest at the mo.
I actually always thought she was happiest in a large group of mares and always hated the idea of her being on her own or with one other, but circumstances (tendon injury) forced her to be on her own all last year and she surprised the hell out of me by how well she adjusted and how happy she was!! Same with being out with just one other horse, although that does cause some problems in that they can get a bit clingy to each other at times.
Hate to say it, but goes against every principle I ever had, but I'm loving having my mare on her own at the moment, makes life much easier for me!!!! Although it wouldn't be reason enough for me to do it forever and I do think horses prefer company.
All seperate - gelding in the middle with a mare on each side. I'm not overly happy with it but Maiden seems to be coping. She will never go in with the gelding again (she has been in with him, with the other mare on her own, and she has been in with the other mare with the gelding on his own) as he attacked her a couple of months ago for no apparent reason. The two mares are quite fond of eachother and if it weren't for our grass situation (long story, we strip-graze and other mare's owner gives them too much and I can't risk mine getting lami) I would have them in together. I would always think twice about a mixed pair or small group after what happened to my mare back in April (she was very distressed about it all). A large group in a decent-sized field, I wouldn't worry so much.