Capacity listed in brackets is required capacity, and marked like so: [1]
Capacity not listed in brackets is available capacity: 1
Either type can be a single number, as above, or a range: 1-3, [1-3]
The specific information for sensor capacity is on p446, in the table named 'Sensors'. Officially, sensors (single or array) may be placed in any device that has the requisite capacity to hold them.