Your friends are dead wrong. Don't take him back.
He didn't cheat on you once. He cheated on you numerous times. And if you take him back, he will continue to cheat on you. It's a no-brainer--just like him. You gave him 12 years of love and loyalty, and this is your repayment?
Good for him for confessing, I guess, although confessions are often more about relieving the cheater's guilty conscience than about doing the right thing. Regardless of the reason, his confession doesn't erase what he's done. He can be sorry all day long, but he's still a cheater and always will be.
It's like what my wife and I tell my daughter over and over again as we try to teach her personal responsibility: actions have consequences. If she gets angry with a Wii game and throws the controller, we'll forgive her and continue to love her as much as always, but she still won't be playing Wii for a while. If you drive drunk and kill someone, you might be sick with guilt and regret for what you've done, but you're still going to jail. Consequences. It's how we learn not to do things that suck.
Your boyfriend might be truly sorry, he might get a gold star for confessing, and you might forgive him for what he's done, but he still must live with the consequences of his actions--losing you as a girlfriend. Forever. Maybe it will teach him something so that he doesn't make the same mistake with future girlfriends.
Don't take him back. Please. This one's over. Move on.