Learn Icelandic: Margir mæta á mótmælin í dag.

In Icelandic, when you are talking about people in general or a mixed group of men and women, you default to the masculine plural. Here, Margir acts as the subject of the sentence ("Many [people]"), so it is in the nominative case.

Why does the preposition á take the accusative case here?

The preposition á can take either the dative or the accusative case. Because the verb mæta (to attend or show up) implies movement or going to an event, á takes the accusative case. If you were already there and just standing around at the protest, it would take the dative.

Is the word mótmælin (the protests) singular or plural?

It is plural! The base word is mótmæli (a neuter noun), which is usually used in the plural when talking about public demonstrations. The -n at the end is the attached word for "the" used for neuter plural words in the accusative case.

If I wanted to say "Few attend" instead, would the verb mæta change?

No, the verb stays exactly the same. Both margir (many) and fáir (few) act as plural subjects, so you still use the plural verb form mæta. You would simply say: Fáir mæta á mótmælin í dag.