.I have just the one Min but he isn't alone. He shares his paddock with our two goats (a Pygmy & a Nigerian) and one side of his paddock butts against the big horse area so he's not totally alone. He also gets out multiple times a week and I spend as much time as I can with him so I don't think he's lacking in socialization.
I would love to get another Mini and some day I will but right now we are saving up for college and to purchase a nice AQHA show horse for my sister so it's not gonna happen for a while. Right now I'm just focusing on working with my Mini and learning all I can so that when I get another I will know what I'm doing and can maybe get a rescue because I love helping the rescues get better.
Anyways, I think if you have just one Mini and some other animal like a goat, large horse, etc that can either share a pasture or fence line with it then they still get the socialization but I definitely think two Minis\Shetlands together is the best. My friend that got me started on Minis used to have just one so he adopted two very small Alpaca babies and the Mini fell in love with them and raised them like horses and they get along fine. I guess it depends on the horse because I know my pinto would HATE being an only horse but the Arab we are boarding here and my sister's pony would love to be only horses.