The debug key should only be used during development. You shouldn't distribute any application signed with this key. In fact Google Play will reject APKs signed with the debug key. See this short guide: Signing your application before uploading to Android market The problem with the debug key...
False. If you code as a hobyist or self publishing dev, then yes, you only need one, BUT if you develop for tiers and publish the app using THEIR publishing account, you either need to create per client keys or have them provide one and sign your app using external tools.
Otherwise the app will not be "updatable" by app store.