He knew a lot more about regeneration than he let on to Rassilon. The truth was, that he didn't want to help Narvin get his regenerations back. He didn't want to open up the gateway that the other Gallifreyans hadn't evolved into, and while he hoped someday those vicious forerunners of his people (that he swore was still reflected in the hate behind their eyes) would be able to possess the ability, they were very much not ready for it.

"I can tell you a bit. I'm afraid I'm not all that well informed."

Lies, lies, and more lies. But they're friendly lies? The sort meant to invite and comfort and not coldly deceive. On to the truths. "We've only thirteen of them. Any more is a risk to our physical and mental health. This is my seventh. They're more difficult to control if you die traumatic deaths."