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There are three different devices to choose from on that Amazon listing.
176°F (80°C): 2W - 5W
110°C: 3W - 10W
220°C: 5W - 28W
The high wattage is the power output at room temp and the higher wattage is the output at the temperature listed. You will still need a temp sensor snd control from the Arduino but the lower power near your set point temp means it is easier to control an exact temp without overshoot.
If you want to add additional units, connect in parallel.
It is not clear if you can submerge them.
No polarity.
You'll need a relay module or a MOSFEt to control this with an Arduino. An Arduino cannot directly output that much power.
The 12v is approximate in most cases. The more stabile the voltage, the more easily you can control the voltage with On/off threshold settings. If you have varying airflow or room temps, you can look into PID algorithms.
You will need to insulate the bath because the lower power listing is not much and you'll lose that much heat to surroundings without insulation or you'll need multiple heaters.