Baseline (Outside) Mold Test

I’m wondering how far away from the subject residence can I take an outdoor reading using and Air-O-Cell canister and machine? I’m going to perform 16 tests on a set of condos that are all attached and close together. Front doors of the farthest two units are maybe 75 feet. Is one test somewhere in the middle sufficient? Maybe two? I’m really hoping I don’t need to take 16 from a time and expense standpoint.