UPDATED 8/28
As things stand right now...
60 Cubs minor leaguers are eligible for selection in the December 2021 MLB Rule 5 Draft, and 45 Cubs minor leaguers (30 post-2021 6YFA and 15 2021 second-contract players) are eligible to be declared MLB Rule 9 (formerly known as MLB Rule 55) minor league free-agents at 5 PM Eastern on the 5th day after the final game of the 2021 World Series (or at 5 PM Eastern on October 15th if the 2021 World Series is cancelled).
In addition, OF Nick Martini was assigned outright to the minors on 6/7, and because he has Article XX-D status (he has been outrighted previously in his career) he had the right to elect free-agency when he was outrighted, or else accept the outright assignment and defer the right to elect free-agency until after the conclusion of the MLB regular season (he had 48 hours to decide).