In most cases there is one CT for each line that need to be monitored as can be seen in the diagram below:
But in the diagram below two CTs are used to monitor three lines. The diagram is for Form 9 and 9S, 3 Wire Wye setup on ION8600 meter. Essentially with 2 CTs, 3 lines can be measured and with 3 CTs 4 lines can be measured. To summarize this, N CTs can measure N+1 lines.
The way the setup works above is that Ia or I1 is measured at I11 and Ic or I3 is measured at I31. The return path for Ia is though I21 and Ic is I32. The two current are combined and fed into I22. The current passes through I21 which acts as a return path for the current and is measured at that point. Inside the meter I11 is connected to I12, I21 to I22 and I31 to I32 etc. See the diagram below for better understanding.