I debated this in my own head when I read the description actually. I came to a conclusion that rather than it simply being a pair per appearance, the text in the adventure could be read as "a pair of Phlegamor's Servants appears whenever someone comes near the gem." Meaning, that it could be a pair per person coming within range.
My PCs average 7th level at this point. So if the CR is low, then a pair won't make for much of an encounter. A pair per PC though complicates the encounters. It doesn't help that the chapters can be run in any order, so balancing for PC level is difficult. As long as the ECL can be run with some middle ground, I can work with it.
I just mention the alternative interpretation in case it helps you guys with the creation process.