Does a single neuron always release the same amounts of different neurotransmitters everytime?

A single action potential, assuming there have not been any action potentials recently, will release the same amount of neurotransmitter, or very close to the same amount, every time. This is why action potentials are called "all-or-none" responses. When the action potential reaches the axon terminal, voltage gated calcium channels will open and facilitate the fusing of vesicles to the axon terminal membrane. Since action potentials tend to remain the same size, the amount of calcium channels that open is similar between APs and thus neurotransmitter release is constant.

HOWEVER there is a caveat. Strong stimuli will typically elicit more action potentials. This in turn will lead to more vesicles being released.

Another important fact: if there is a series of action potentials that follow each other very rapidly, the later action potentials will release less neurotransmitter than the first action potentials. The reason for this is that there is a finite amount of neurotransmitter carrying vesicles at the axon terminal and it takes a little while for new vesicles to be filled. Action potentials can occur with frequencies up to 500Hz in some cells (rare, but it can happen), which would mean there would be an AP every 2ms. A long string of these, and by the end the vesicle pool would be almost depleted. It is through this mechanism that single action potentials can release different amounts of neurotransmitter.

Finally, you are right, the information carried by neurons is primarily the NUMBER of action potentials that are fired. Very little information is carried through the amount of neurotransmitter is released. There is also some debate in the literature as to whether the TIMING of the individual action potentials carry information, but that is an answer for a different question!

Hope this helped.

You may be thinking of the idea that the time duration, not the amount, of transmission varies