There are many factors to consider when determining how much space chickens need, but generally a minimum of 5 square feet of indoor space per bird and at least as much outdoor space is a good start.
Factors to consider when determining the amount of space needed include age, health issues, activity level, climate, and type of outdoor space. For example, a flock of elderly birds with mobility issues will likely need less space, especially outdoor space, than a younger, more active flock. In colder climates, a smaller (but still well ventilated space) will be easier to maintain at a comfortable temperature than a larger space but may be too warm for that same number of birds in a warmer climate.
In terms of outdoor space, birds living in areas with little vegetation may benefit from larger outdoor spaces than birds who have access to outdoor spaces rich with vegetation and things to explore.
You’ll need to pay attention to the flock- if birds are picking on each other, or it seems they don’t have enough room to get away from each other, they likely need more space.