Rats do best in groups of two or more. Lone rats can get very lonely and even suffer through a type of depression. It's no extra cost or work to have 2 rats .
Siam is my little rescue rat who was kept alone from the time she was 6 weeks old until I got her (at 8 months old), and you could see the joy on her face when I introed her to Amelia and Willow. She's the first into ratpiles now, and is always following or playing with the other girls.
Also, the bond between opposite sex altered cagemates can be even more strong. Tsume, my old neutered man, was wll loved by his ladies and loved them back just as much. There's something about a pond and a half of squishy manrat that is just irresitable to a lady .
Well, I know that this is going to be controversial... But when I got my first rat, she lived alone for 8 months before I found a friend for her. I had her out of the cage for 3/4 of the day, and at night she ran on her wheel like a maniac. She always seemed perfectly happy, between the wheel, elaborate nest building, and her cage toys. At 4 months, I started taking her for regular playdates with a friend's girls, and they would happily tussle, chase, and hump each other. Now she finally has a permanent companion, with whom she sleeps & hangs out with only a tiny portion of the time. Honestly, I see no difference in my rat's personality. She seemed very content when she was alone, and just as content now that she's not.
However, I do realize that this is the exception to the norm. My girl had the luxury of being played with by a human constantly, since I'm a college student, have a loose schedule, and can easily study while using one hand to absentmindedly play with her. If she hadn't had so much attention, she probably would not have been such a happy, well-adjusted rat.
Granted, I'm not advocating having single rats. But in situations where they get excessively large amounts of attention, and have enriched cage environments, a single rat can be happy.
Of course, then you do miss out on the cuteness factor of rattie piles, gigantic multi-rat tussles, simultaneous wheel running, and silly hoarding behavior. Rats are definitely more interesting & cute in groups... especially 5+. Once you've experienced rat's awesomeness in groups, your biggest problem will be limiting the number you have.