As someone who has owned plenty of rats, I have known people who have had pet rats that do fine by just themselves if that is something your bent on doing. Whether you get one rat or two, you should spend as much time with them as you can- 30 minutes, 2 hours, whatever you can. They are very social, and are just fine wandering around in a safe area while you watch over them while you do homework, dishes, whatever.
However, nothing replaces the loving, constant companionship of another rat of the same gender as a cage mate. They groom one another, sleep and play together, and form a deep bond.
Whatever you decide, having more than one won't make them love you any less. : ) I think you're making a great choice- rats are, in my opinion, the best rodent pet one could ask for!