As escolas de samba desfilam com três ou quatro casais de porta-bandeira, mas apenas o primeiro deles é avaliado.
So, there’s usually one couple that is the standout that takes the first position. The movements of the porta-bandeira – always a couple, are always stunning.
Look at how ‘deles’ is used here.
Deles = de + eles == of them
So the sentence is saying ‘…just the first of them (of the couples) is evaluated.’
Let’s see another example of dele :
Você conhece o pai dele?
And with ela it’s de + ela = dela:
Você gosta do cabelo dela?
And the plural them, eles used in our first sentence above. Notice that it’s a man + a woman, You know that this is, by default ‘eles’ (masculino), right?! It would be delas only if we were talking about two or more women!