Learn Spanish (Spain): Cuanto más vamos al muelle, más nos relajamos. - The more we go to the pier, the more we relax.

In English, we use 'The more... the more...' to show that two things increase together. In Spanish, you cannot literally say 'El más... el más...'. The correct grammatical structure is cuanto más (literally 'how much more') followed by the first action, and then just más followed by the resulting action.

Does the word Cuanto ever change its ending here, like to Cuantos?

In this specific sentence, it does not change because it modifies a verb (vamos). When cuanto más describes a verb, it acts as an adverb and never changes form. However, if it were describing a noun instead of a verb (like 'the more days'), it would change to match that noun (cuantos más días).

Why do we need the pronoun nos before relajamos?

The verb used here is relajarse (to relax), which is reflexive when you are talking about relaxing yourself. If you just said más relajamos, it would sound like you are relaxing someone or something else. The nos shows that the action of relaxing happens to yourselves.

Where would I put the subject nosotros if I wanted to include it?

In these proportional comparisons, the verb and its pronouns (like nos relajamos) usually stick immediately after más. If you wanted to emphasize 'we' by adding nosotros, it sounds most natural to put it after the verb: más nos relajamos nosotros.