A “peg count” is simply a count of the number of calls that have taken place during the period of time being measured. It is an old term going back to the days of cord boards, and was a term used way back when A.K himself came up with his famous formulas for traffic calculation. You can’t interchange it with the number of lines in traffic calculations.
If you want a good description of traffic theory for non-math people, check out “Telecommunications Mangagement” by James Harry Green. He does a good job of laying it out. He even includes the formulas for the various traffic calculations everyone keeps asking for here. Though if you aren’t into stats, they won’t do you much good.