Addition Hangman is a game for two players. The rules are
The first player (the Chooser) chooses an addition problem $x+y=z$, where $x$, $y$, and $z$ are positive integers. He writes the addition $x+y=z$ in base $10$, replaces each digit by a blank space ($\underline{\hspace{8mm}}$), and shows the result to the second player.
The second player (the Guesser) starts off with some number of tokens. She may risk a token in order to guess a digit. If the guessed digit appears anywhere in the addition, the Chooser fills in the appropriate blank space(s), and the Guesser keeps her token. If the guessed digit does not appear, the Guesser loses her token.
The Guesser wins if all of the blank spaces are filled in. The Chooser wins if the Guesser runs out of tokens.
Question: The Guesser receives the addition problem $$ \begin{array}{cc} &\underline{\hspace{8mm}}\\ +&\underline{\hspace{8mm}}\\\hline &\underline{\hspace{8mm}} \end{array} $$ Each blank represents a (not necessarily distinct) digit between $1$ and $9$. What is the smallest number of tokens the Guesser needs in order to be certain she can win?
$\newcommand{\bl}{\underline{\hspace{8mm}}}$