Ask the festival if you can send a poster in advance for hanging, and ask how many they need (usually 1-2). I don't think you will need more than 3. One to send in advance, 2 spares, to hang where you see other people hang theirs. If they are cheap, get 5 but no more (you may be able to hang a couple in local shops but I'd reserve this to a one block radius.)
Postcards are usually going to be a 500 or 1000 minimum correct? Just figure out how many you will need. Leaving small stacks at the theater and around town, handing out, etc.
It all depends on your attack plan.