Basically once, but it could happen in more than one way.
First off, you should do a drawing yourself (or just pick your preferences) before the campaign starts. This way, even if the players never do an in-game drawing, you still know where the treasures are, who the ally is, etc. Because the drawing isn't guaranteed to happen in-game at all. If it does happen, your "private" drawing gets replaced/updated.
The most likely place the drawing will happen is with Madame Eva at the Tser Pool Vistani camp.
If it doesn't happen there, the adventure provides a "back-up" option of having Ezmerelda do it after the party meets her (she can be introduced in any of several different locations, at the DM's preference).