You could start with the MLX90614, that's hard enough to detect the number of people.
The temperature you measure depends on the temperature around. On a cold day it is easier to detect people.
I suggest to combine a few sensors.
At least add a PIR motion sensor. You could even use two of them, on the left and right side of the art object. You could use more PIR sensors, like a row of 5 or 7, and shield the front with a small opening for a small beam.
Ultrasonic sensors can measure the distance. For example the ping))) modules. Perhaps you should use just one. I don't know if the ultrasonic sound will interfere if you use more than one.
Some radar motion detectors are not expensive. But you need the complete module with variable sensitivity, not the radar elements without amplifier (as sold on Ebay).
A theramin is something that senses if someone is very close : http://en.wikipedia.org/wiki/Theremin
Measuring the weigth of the people in front of the art object with a weigth sensor in the floor is out of the question ?