How mush the matter need to convert to plasma?

Question

How mush the matter need to convert to plasma?

Answer 1

A single atom.  

Plasma is primarily an ionized gas.  So for example, you take a single atom of hydrogen and heat it up to the point it throws off its electron, you've turned it into plasma.

Answer 2

There are certain criteria for an ionized gas to be considered a plasma. While generically an ionized gas is a plasma there are some textbook definitions of plasma.

The debye length should be much smaller than the size of the plasma:

This is because a property of a plasma is that is can shield an electric field, since the debye length is the characteristic shielding distance, if the plasma was smaller than that it wound not be able to shield the electric field.

The average kinetic energy should exceed the electrostatic potential energy between particles: If it doesnt the plasma is considered strongly coupled and it doesnt act like an ideal gas anymore and standard plasma theories dont work.

The electron-neutral collision frequency should be much smaller than the plasma frequency: If there were more frequent collisions with neutrals, that would dominate the plasma behavior and it wouldnt be in equilibrium over any useful length scale. The plasma frequency is used to determine a lot of things about the plasma so it needs to be dominate or using it in a definition is useless.

So,  all these 3 things depend on the density and/or temperature of the plasma. So the amount of matter isnt the question it is the amount of matter/volume that is important.

I have to disagree with the answer that only 1 particle can form a plasma, many particles are needed inside a debye sphere for shielding to occur.