участников

участников

"Participants."

In this sentence, "участников" refers to participants in general and is in the genitive plural form because it's governed by the implied quantifier of "количество."

Количество участников растет с каждым годом.

The number of participants grows every year.

In this example, "участников" is in the accusative plural form because it is the direct object of the verb "видел."

Ты видел участников на фотографии?

Did you see the participants in the photograph?

Here, "участникам" is in the dative plural case to indicate the indirect object receiving the explanation.

Маршрут был описан всем участникам перед началом.

The route was explained to all participants before the start.