I've seen a number of threads recently where posters are asking about wireless issues, and are trying to get a good result... but a number of times, I saw the number of units being used as an almost (to me anyways) determining factor. So, I thought I'd share an experience I had this past weekend as an example.
We played three different events in a row in a venue along the Maine coast (beautiful venue, beautiful view, beautiful weather, great clients...) It was really tons of fun. The A/V tech at the venue and I worked to make sure that my wireless transmitters were off during their ceremonial portions of the event. They were using three wireless units on the 2nd & 3rd days. All their wireless mics are in the 500MHz range as are our IEM transmitters.
On day three, we had a little extra time, and the A/V tech provided me with the 6 frequencies that were already set when he took over. I used my IAS and found direct hits on two of the units, and this was while I was entering the frequencies in use and had only gotten to frequency #4, IIRC.
So, in the spirit of collegiality, I ran a wireless coordination for him. They're using Shure UHF-R gear (Receivers are UR4D+.) The coordination yielded 21 concurrent frequencies that were available.
Moral of the story: The 'sum & difference' physics of RF can bite you with real unhappy results even in a setup with only a few (5 or so) units.
One other thing: We played a commencement dance at one of the colleges here in Maine, and I'm glad I didn't just think "Oh, there's not been any new DTV activity up here" as when I checked against the FCC database in the IAS I had three direct hits from nearby TV stations and had to change three of our mics - two for the lead singers.
I'm thinking that even for small operators like myself, if you want to play in the RF sandbox, minimal things to have are: Coordination software (free or otherwise,) RF scanners (we use the TTi PSA1301T which covers from 150KHz to 1300MHz,) and a WiFi scanner that shows all the 2.4 & 5GHz activity. That seems to be what I need to survive.