Current regulations mandate a specific number of samples for each distinct or “homogeneous” material type.
The general guideline follows the 3, 5, 7 rule:
- Less than 1,000 square feet of the suspect material: 3 samples
- Between 1,000 and 5,000 square feet of the suspect material: 5 samples
- More than 5,000 square feet of the suspect material: 7 samples
For instance, if there is less than 1,000 square feet of popcorn ceiling, only 3 samples would be required, and so on.
The regulations actually recommend 9 samples per material type (and sometimes less), see EPA Pink Book and here. But local AQMD regulations unofficially enforce a minimum of 3 per material type (see bottom of page 14 here).