Learn Turkish: Sekiz mühendis zengin değil. - Eight engineers are not rich.

In Turkish, whenever you use a number before a noun, the noun stays in its singular form. You say sekiz mühendis (literally "eight engineer"), never sekiz mühendisler.

How do we know sekiz mühendis means "eight engineers" and not "the eighth engineer"?

Regular numbers for counting (one, two, three) use just the plain word, like sekiz. If you wanted to say "eighth" to show an order, you would add a special suffix to the number to make it sekizinci.

Why is değil used here instead of a negative suffix?

We use the separate word değil to say "not" in sentences that don't have an action verb. Because we are describing the engineers with the adjective zengin (rich), değil is the right choice. Action verbs get a negative suffix instead.

Does değil need a suffix to show it means "they are not"?

It doesn't have to have one. For "they" (third-person plural), the sentence can just end with the plain word değil. You could also add the plural suffix and say zengin değiller, but leaving it off is very common and perfectly correct.