Three.
There they were, standing in a row, like a perfect team. They weren't quite equal; the Doctor was standing a little in front of Jack, and Rose, in the middle, was slightly behind both. But they fit, the three of them.
He looked at Jack, grinning prettily for no apparent reason, and then Rose, who met his eyes with warmth and encouragement and a tiny beautiful smile of her own. She held out her hand to him, and he felt his heart soar as he took it.
He flashed a grin at Jack and took a deep breath. "Ready?"
"Ready."
"As always, Doc."