the problem with crowdsourcing is that it would probably be a pre-publication which would make it difficult to submit the story elsewhere because many magazines (including clarkesworld) do not allow that.

currently they have multiple slush readers, and depending on volume they could engage more as long as the volume does not grow indefinitely.

btw, here is a thread that gives some insights how slush reading works: https://www.fantasy-writers.org/forum/slushpile-answers-clar...