Learn Polish: Najwyraźniej trudno oszacować ten tłum, ale to około pięciuset osób. - Evidently it's hard to estimate this crowd, but it's about five hundred people.

The word około (about/approximately) always requires the genitive case. Because of this, the numeral pięćset (five hundred) changes to its genitive form pięciuset, and the noun osoby (people) changes to the genitive plural osób.

How would this sentence change if I used the particle z to mean 'about' instead of około?

If you used z for approximation, you would say z pięćset osób. Unlike około, using z to mean 'around' or 'roughly' requires the accusative case. For numbers like five hundred, the accusative looks identical to the nominative, so the phrase doesn't change its endings.

Why is it trudno oszacować rather than jest trudne oszacować?

In Polish, when you want to say 'it is [adjective] to do something', you generally use an impersonal construction. You skip the verb 'to be' and use the adverb form of the word. Therefore, you use the adverb trudno directly followed by the infinitive oszacować (to estimate).

Is the word najwyraźniej related to the word wyraźnie (clearly)?