The answer is that it is uncountable. Here we will explore why and how we can use 'furniture' in its countable form.
Why is 'furniture' uncountable?
In both pictures we can say, correctly: 'There is furniture in the room.'
The room on the left has more furniture than the one on the right but, because furniture is uncountable, we can say 'there is furniture'.
To make our sentence more accurate we can add determiners: a lot of, lots of, some, a little or no / isn't any. However, there will always be 'furniture' - uncountable.
There is a lot of furniture in the room.
There is lots of furniture in the room.
There is some furniture in the room.
There is a little furniture in the room.
There is no furniture in the room.
There isn't any furniture in the room.
The reason 'furniture' is uncountable is because furniture is the collective name for a group of nouns. Furniture includes tables, chairs, sofas, beds, wardrobes, bookshelves, etc. All of these are countable (one table, two tables, three tables). When you have a group of different items together (one table, two chairs, three sofas) it makes the uncountable group - e.g. furniture. For example:
One chair = furniture
One chair + one sofa = furniture
Two chairs + two sofas + two tables = furniture
Three chairs + two sofas + two tables + one bed = furniture
How do you count furniture?
We count furniture using the countable form of 'piece(s)'. Let's see what that looks like.
There is one piece of furniture.
There are two pieces of furniture.
There are three pieces of furniture.
Examples of incorrect sentences
I want to buy some furnitures.
Those furnitures look amazing.
That room had four pieces of furnitures.
I like rooms with little furnitures.
Examples of correct sentences
I have to buy some new furniture for my room.
She likes old furniture.
My parents have too much furniture in their house.
Art-deco furniture is my favourite.