furniture (n) 家具
I have many furniture.
{Furniture} is an uncountable noun. We use {much} to modify it.
I have a lot of furniture.

There are many furniture.
The same reasons mentioned above.
There are many furnitures.

The furniture is expensive.
Ok!

canvas (n) 帆布
The canvas can make bag.
{Bag} is a countable noun, so please use plural form here.
The canvas can be used [to make] bags.

Canvas bag are normal.
{Bag} is a countable noun, so please use plural form here.
Canvas bags are beautiful.

The canvas bag is big.
Ok!