Yes, definitely two minimum. I tried introducing a singleton, but only did it once as it was so difficult for her and she never really got accepted by the other two I had at the time. I've found that, if you give them some time together separately, a new pair will form a bond which supports them through the integration period, and they will then go around together as 'best friends' thereafter.
From the health point of view, it's undoubtedly best to give complete quarantine for several weeks, as Alex says, but for many of us this isn't very possible, for reasons of space etc. I've maybe been lucky so far, but I've found that it's been OK to divide my run with netting and provide a temporary coop for the newbies, so they can see the older hens and get to know them a bit through the netting. This has worked OK for me so far because I've always bought direct from a known and trusted breeder, rather than eg from a market or sale, where the birds will have been subjected to extra stress, lowering their immune systems, and making it more likely that any disease they've picked up in their unknown backgrounds will emerge.