Okay, smørbrød is both singular and plural. and because a number is not specified then you assume that it's plural.
In the same way that: "I have a sheep" means one sheep
And, "I have sheep" Means more than one sheep.
Note for later, jordbær is also both singular and plural.
An open, or open-faced, sandwich is a sandwich consisting of only one slice of bread with toppings on it - not two slices with a filling in-between.
When Norwegians talk of smørbrød, they usually mean the former, but when English-speakers talk of sandwiches they're usually referring to the latter.