Maximum number of entries for the Challenger part of the competition is 3 skaters in each discipline.
I also don't know why not sending skaters to Challengers would be a criticism of USFS - This is not Skate Canada, they're not that stingy with assignments, even some of the "lower" ranked women got a Challenger assignment last season and almost always fill all of their spots at US-held Challengers. Plus, we have to consider that this is really early in the season - Some skaters might not be ready yet, and I'm honestly not sure we can expect very high scores overall at Cranberry Cup.