The plural of stuff is also stuff. Stuff is always a plural noun. Stuffs is not in common use.
Stuff is a synonym of things.
Gather up your things
Gather up your stuff.
What are those things over there?
What is that stuff over there?
Stuff means a collection of things(a collective noun). For this reason, it is never singular. It is always plural.
Why is there no s for plural stuff?
Stuff is an uncountable noun. This means that you cannot count the objects that you want to talk about when you use stuff. In fact, this is exactly when we use stuff!
I have a lot of stuff in my house.
There is a lot of stuff in my fridge.
We use stuff in these examples to show that there are a lot of objects(we don’t know how many!).
How to use stuff in a sentence
Stuff is a collective noun so we need to use the third person singular.
This stuff is excellent.
This stuff has a great taste.
When can you use stuffs?
You can use stuffs when it is part of the compound noun foodstuffs.
It is also the third person singular of the verb to stuff(present simple form)
She stuffs the money into her handbag.