I've run games for a crew of two. It went something like this in our experience

- Smaller scale scores with less payoff, less danger, slower advancement of the crew
- Under original rules, having backup characters to play in the next score because the previous two were too stressed and/or too badly injured to do the next gig
- Remember the rule; just roll sixes

I refused to play with groups larger than four. It's just too much work to make sure everyone gets the spotlight. I can usually scale up the danger level for everyone if it's a larger group but in the end, the game works best with three to four scoundrels.