Learn Italian: Il volontariato aiuta molte persone che meritano altrettanta attenzione. - Volunteer work helps many people who deserve as much attention.

Here, altrettanta is acting as an adjective describing the noun attenzione (attention). Because attenzione is a feminine singular noun, the adjective must change its ending to match. If it were used as an adverb to describe a verb (e.g., "he studies as much"), it would stay in its invariable form: altrettanto.

Could I use così tanta instead of altrettanta here?

You could, but it changes the nuance slightly. altrettanta attenzione means they deserve as much attention (an equal amount compared to something else, perhaps the effort the volunteers are putting in). così tanta attenzione just means they deserve so much attention, without implying a direct comparison.

Why do we need the article Il before volontariato when English just says "Volunteer work"?

In Italian, broad categories, abstract concepts, and generalizations almost always require a definite article (like il, la, or lo) when used as the subject of a sentence. English usually drops the article for these general ideas.

Why do we use che for "who" instead of chi?