Learn Russian: О скольких новых тенденциях рассказал этот социолог? - How many new trends did this sociologist talk about?

The preposition о (about) requires the prepositional case. Just like adjectives, the word сколько (how many) changes its ending depending on its role in the sentence. In the prepositional case, it becomes скольких.

Normally, words after сколько are in the genitive plural. Why are новых тенденциях in the prepositional plural here?

This is a key rule for quantifiers! When сколько is in the nominative or accusative case, it forces the following noun into the genitive. But when сколько is in an oblique case (any other case, like the prepositional скольких here), the adjective and noun simply match it. So, all three words are in the prepositional plural: о скольких новых тенденциях.

Why is the preposition о used instead of об, considering the next word starts with two consonants?

The preposition об is only required before words starting with a vowel sound (а, о, у, э, и). Even though скольких starts with a heavy consonant cluster, its first sound is the consonant с, so we stick with the standard о.

Why use the perfective verb рассказал instead of the imperfective рассказывал?