Onlar çok zengin. They are very rich. Thank you

Yours is correct. You can also say:

Onlar çok zengindir.

But I am not sure if we can say "Onlar çok zenginlerdir or zengindirler?"

Please don´t get confused with it! Yours is correct... I am just having doubts with plural form...

I think that zenginlerdir is pure nonsense...

Sonbahar

What does the suffix -dir mean?

dir, dır, dür, dur... play the role of the english "is/are"  especially for o and onlar

It is not nonsense. 

This version makes it sound like a guess : (Probably/I guess/I think) they are rich

Very interesting!! Thank you dilliduduk!

But is zengindirler also correct? Merak ediyorum şimdi

yes it is. but I am also confused now thinking if it is exactly same as zenginlerdir Actually I think they are, my previous explanation might be a bit wrong, usually when you add "-dir" it gives this "guess" meaning. hmmm. Oh my God I am thinking about my own language

and for plurality: 

when the subject is plural and referring to a human, the verb can be plural or singular. 

My students are clever.

Öğrencilerim zeki/zekiler/zekilerdir/zekidirler.

You can use all.

when the subject is not a human, the verb has to be singular.

birds are flying.

kuşlar uçuyorlar.

The plural form can only be used when there is a figurative expression where you address to animals/objects as if they were human.

"Doubt is the path to certainty"...

Thank you for dispelling my doubts!!