I'm not sure it rates as a countable noun. We don't usually use a number in front of 'pressures' in the way we would with cars or children. It's probably better to think of this as a collective noun. Pressures are usually dynamic, meaning they change from moment to moment, and as soon as they change, that's a different pressure. But there are a range of pressures, physical and in a tank of compressed gas or a machine or in tyres. and there are other types of pressure such as stresses and obligations to work and family and society. We don't usually single out a pressure but tend to talk about them collectively.