Wanda's suggestions to Wicked were viable ways of approaching the subject, and Dr. Strange just smiled serenely and looked on, letting the young magic users puzzle out their own paths. There was no one true way about anything; they each sourced their power differently, they could all discover it differently. What mattered was that they were thinking about it, and considering their multitude of choices or even discovering a unique one.
"There is no limited supply, son," he answered Billy with a hint of subdued amusement. "You would have to study with Wanda to find whether it was draining to her, but the source is, until the last star has gone out of the sky, endless."