I don't believe there is a minimum threshold before you can apply for an Adsense account but there is however a requirement that your site(s) feature substantial content. Without enough substantial content your site is unlikely to pass the review made by the Google adsense team. If you are a start up it may be possible that your site won't have the substantial content necessary for passing the review process.
No traffic requirement, as long as your website is quality site and not against any adsense TOS, then you will get approved even though you have 0 traffic