Learn Italian: Marco dona una cifra e io dono altrettanto. - Marco donates a figure and I donate as much.

In this sentence, altrettanto is acting as an adverb modifying the verb dono (I donate as much). Because it is an adverb here, it is invariable, meaning it never changes its ending. It would only change its ending to agree with a noun if it were used directly next to one as an adjective (for example, altrettanta pazienza - "as much patience").

What does una cifra mean in this context?

While cifra usually translates to "digit" or "number," in financial contexts like this one it refers to "a sum of money" or "an amount." You can think of it just like saying "a figure" in English.

Could I say dono la stessa cifra instead of using altrettanto?

Absolutely. Dono la stessa cifra ("I donate the same amount") means the exact same thing. However, using altrettanto is a very natural and concise Italian way to express "as much" or "the same amount" without having to repeat the noun.

Since the verb dono clearly means "I donate," why include the pronoun io?