There are guaranteed to be at least 2. My guess is 4.
Assuming either BDB or Vanguard Cadets are the highest placing Open corps in Semis, how high is the ceiling? With this year's World corps, could either crack the top 15?
They compete under the international rules set. They can make Open Class Finals, but cannot displace another corps to do so. For example, if they make finals, Open Class would have thirteen in finals instead of the normal twelve.
I'm not sure how the rules are applied when it comes to World Class prelims/semis, but I'd guess it would be a similar situation. If they make it to Friday, DCI would likely just add an extra corps.