Learn Spanish (Spain): Tenemos demasiadas cajas; ya hay suficientes. - We have too many boxes; there are already enough.

In this sentence, it describes the noun cajas (boxes), so it acts as a determiner. This means it must match the gender (feminine) and number (plural) of cajas. It only stays as the invariable demasiado when it acts as an adverb modifying a verb, like in hablamos demasiado (we talk too much).

Why is there an 's' at the end of suficientes?

Here, suficientes acts as a pronoun replacing the noun cajas. Because it refers to multiple boxes, it must be plural. Note that unlike demasiado, the word suficiente only changes for number (singular or plural), not for gender.

Could I say suficientes cajas at the end instead of just suficientes?

Yes, you could say ya hay suficientes cajas, where suficientes acts as a determiner describing the boxes. However, dropping the noun and just using suficientes as a pronoun is much more natural because cajas was already mentioned in the first half of the sentence.

What is the purpose of ya before hay?

The word ya means 'already.' Saying ya hay suficientes (there are already enough) emphasizes that the limit has been reached, making it perfectly clear that you don't need or want any more boxes.