The smallest number of molecules that we can have inside a container is zero. In that case, we have a vacuum and our pressure is zero. But we can never actually have negative pressure. Now, technically, what they mean when they say negative pressure is a negative pressure difference and what a negative pressure difference is, when we subtract the high pressure and the low pressure, we get a negative value because the change in pressure values is a negative quantity. So you should be careful in using this term negative pressure because negative pressure doesn't make sense. But negative pressure difference does, in fact, make sense.