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These numbers indicate how many open missions are there in the map for that character to start.
This seems to include main story missions as well as 'Question Mark' missions only available for that character.
It does not include random encounters not visible on the main map as well as pastime activities like races or sport events.
I just finished evaluating the jewelry I want to rob, as well as choosing my crew and how to rob the place. Now, I'm on my way to steal a Pest Control van, when suddenly that programmer from Life Invader whom I helped with his pop-up virus problem, called me and asked me if I could get him a job.
Now since the heist is already planned, I was curious if I can't switch my current hacker with that guy, after I evaluate his skill and the share he wants.
Is there a way to do this, and how?
I don't think there is a way to change the crew once it's been selected, unless you have a game save before you made the selection that you can revert to. I was unable to change the crew when I tried.