If all the baseboard heaters are powered from the same pair of 220V 30 Amp (or less) circuit breakers it can be done with one EZSwitch30. Rather than running power to the baseboard heaters directly, the circuit breakers run 220V power to the EZSwitch30. The EZSwitch30 requires a Neutral connection as the Insteon logic runs off of 120V. The 220V switched output of the EZSwitch30 goes to the baseboard heaters. If the baseboard heaters are on multiple pairs of circuit breakers than an EZSwitch30 is required for each pair of baseboard heater circuit breaker pairs.
The existing thermostats are powered from some electrical circuit. If it is the same 220V circuit(s) that is/are providing 220V power to the baseboards then power to the thermostats will be lost when the EZSwitch30 turns the 220V power off. This would remove power from the Venstar Thermostat which is not what you want. In this case the EZSwitch30 would be installed at a point after the thermostat connection, before the contactor that is now controlling the baseboard heaters. Just as you do not want to remove power from the Thermostats when the EZSwitch30 is Off, you do not want to remove power from the EZSwitch30 when the Thermostats are not calling for heat.
I cannot offer any advice on whether the Venstar Thermostat can replace your existing Thermostats. That would be a question from Venstar or Smarthome.