You want 4 probes if:

• You are monitoring a 3 phase Wye or 4 wire delta system (3 phases plus neutral) and you wish to measure neutral current.
• You are monitoring any 3 phase system and you want to have an extra current probe that can be used on one of the phases.  An extra probe is added insurance that you will have success.
• You are monitoring 4 parallel single phase loads (neutral current probe is used for the 4th load)
• You want to measure 4 DC branch loads (i.e. sharing the same voltage source) simultaneously.

You want 3 probes if:

• You are measuring the power of a 3 phase delta and don’t want to have an extra probe available, just in case.
• You are monitoring a split-phase power system (e.g. 120/240V) and want to measure neutral for leakage or attached single phase loads
• You are measuring 3 phase Wye or 4 wire delta and want a different type of probe for measuring the neutral (e.g. a lower capacity probe for measuring imbalance or leakage)
• You are measuring power of 3 parallel single phase loads
• You want to measure 3 DC branch loads (i.e. sharing the same voltage source) simultaneously.

You want 2 probes if:

• You are measuring the power of a split-phase circuit (e.g. 120/240V)
• You are employing "2 current power mode" to measure total power of a 3 phase load
• You are monitoring a high power load via 2 installed CTs (Current Transformers).
• You want to measure 2 DC branch loads (i.e. sharing the same voltage source) simultaneously

You want 1 probe if:

• You are measuring a single phase load
• You wish to use a special probe for neutral imbalance or ground leakage current
• You believe all phases are balanced and you want a rough measurement of power.
• You want to measure a DC load.