Around 1000 watts is a typical starting point for a 100-sq.ft. room under standard conditions. Poor insulation, large windows, high ceilings, or colder climates can increase the required wattage.
Yes, one properly sized baseboard heater may be enough for a 100-square-foot room. However, the actual requirement depends on the room's amount of heat loss, insulation, windows, ceiling height, and outdoor climate. Before installing more than one heater, make sure the electrical system is properly sized to handle the circuit, thermostat, wiring, and combined electrical load of all installed devices.