My quick rule to start with is to double the number of starters. So in this example, 22.
Beyond that, it depends on what options your league is using. For instance, in a dynasty league I would add additional spots, maybe push it to 25 or 26 spots. That way you can keep prospect, i.e. a young QB like Deshaun Watson or Mitch Trubisky.
Or, with an easy league, you may want bigger rosters. This keeps the owners from having to grind during bye weeks.
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