Learn Arabic (Modern Standard): لو وصل الوباء، لمات كثير من الناس. - If the epidemic had arrived, many people would have died.

In Arabic, hypothetical or unreal conditions (something that didn't actually happen) are introduced by لو (if). To show that the condition is unreal, Arabic simply uses past tense verbs in both the "if" clause (وصل) and the "then" clause (مات). You don't need complex auxiliary verbs like you do in English.

What is the function of the prefix لـ in the word لمات?

That is the conditional response particle. When you have a hypothetical "if" sentence using لو, and the result (the "then" part) is an affirmative action, you attach لـ to the beginning of the result's verb. It emphasizes the outcome but doesn't have a direct English translation.

Could I leave out the لـ and just say مات كثير?

While you might hear it dropped in casual spoken Arabic, in Modern Standard Arabic, adding لـ to an affirmative result clause after لو is the standard, correct grammar. If the result were negative, however, you would not use لـ.

Why is "many people" translated as كثير من الناس?

In Arabic, to say "many [noun]" or "a lot of [noun]", it is very common to use the word كثير as a standalone noun (meaning "a large amount") followed by من (of) and the definite plural noun (الناس).