Low, middle and high poly

It all depends on the model how many polygons.

Low poly is generally low detailed, even blocky in style

High poly is highly detailed, with a higher polygon count.

You can create a high poly model and a low poly model and bake normal maps onto the low poly.
This method is often used in games to show detail whilst keeping the game running smooth and efficiently with low poly models.

For arc viz, avoid low poly, look to add the details whilst maintaining some level of efficiency.
Go for detail, use the necessary number of polygons to get the job done.

Here are three spheres



The one on the left has 128 faces
The one in the middle has 512 faces
and the one on the right has 2048 faces
The one on the left would be unacceptable for a door knob for example in an arc viz render.
The middle is acceptable and efficient.
The one on the right is overkill.

Like Adrian said, it depends on the model.

And:

nobody (I think) is counting polygons to decide whether something is low-poly or not.

You can also have a good idea about how low, middle or high poly your model really is when you judge it by its wireframe. It's easy to see what is what by how dense your object seems.

At least an approximate number of polygons, please

In conclusion, there is no approximate number of polygons to define low or high poly, the number of polygons is relative to the model.

Think,
Low Poly = low levels of detail, blocky shape and style
Med Poly = more detail, smoother shapes, efficient enough for good quality games
High Poly = high levels of detail and more realism