More than you would think, but focusing on this is something that people who aspire to build companies do to avoid actually building companies. You're almost always better off writing software or talking to users.
Who's the #1 person you could hope to get the word out in the world? Wave a magic wand and get them, and you'd probably find that this gets you zero additional users at the margin.
You are almost always better off talking to your existing users and getting their honest feedback about the product than trying to get a celebrity endorsement.