LAST UPDATED 10-21-2017 (after conclusion of NLCS)
As things stand right now...
At present, 19 Cubs minor leaguers are eligible to be MLB Rule 55 minor league free-agents at 5 PM Eastern on the 5th day following the final game of the 2017 World Series, and 58 Cubs minor leaguers are eligible for selection in the December 2017 Rule 5 Draft.
So the Cubs have until five days after the final game of the World Series to either sign a post-2017 minor league free-agent to a 2018 minor league successor contract or add the player to their MLB 40-man roster, otherwise the player will automatically be declared a free-agent.