Learn Arabic (Modern Standard): هناك عشرون مشاركا في المؤتمر. - There are twenty participants in the conference.

In Arabic, when counting from 11 to 99, the noun being counted is always singular! It also takes the accusative case, which means it gets an "-an" ending. That is why the sentence uses مشاركا (literally "a participant") instead of a plural form.

Why is there an extra alif (ا) at the end of مشاركا?

Because it is the counted noun after a number from 11 to 99, it must take the accusative case. For indefinite nouns that end in a regular consonant, the accusative is marked by the "-an" sound. In Arabic spelling, this "-an" ending requires adding a silent alif (ا) to the end of the word.

I sometimes see "twenty" written as عشرين. Why is it عشرون in this sentence?

The words for "tens" (20, 30, 40, etc.) change their ending depending on their job in the sentence. Here, "twenty" is the subject ("There are twenty..."), so it takes the default nominative ending -ون (-ūna). If it were the object of a verb or came after a preposition, it would change to the "-īna" ending (عشرين).

Does هناك change to match the plural number of participants?

No, هناك stays exactly the same! It translates to both "there is" and "there are." It is a fixed word and does not change its form regardless of whether the noun that follows is singular, plural, masculine, or feminine.