Learn Spanish (Latin America): Sintiendo empatía por el prójimo, ayudaremos a cuantos sufran. - Feeling empathy for our fellow human beings, we will help all those who suffer.

We use the subjunctive sufran because we are talking about an indefinite or hypothetical group of people. We do not know exactly who these suffering people are yet; we are just saying we will help whoever they might be in the future. If we were referring to a specific, known group of people who are currently suffering, we would use the indicative sufren.

Could I use todos los que instead of cuantos in this sentence?

Yes! The word cuantos acts as a universal relative pronoun here, meaning 'all those who' or 'as many as'. You could perfectly well say ayudaremos a todos los que sufran. Using cuantos just sounds a bit more literary and concise.

Why is there an a before cuantos?

That is the personal a. Because cuantos refers to people (those who suffer) and functions as the direct object of the verb ayudaremos, the personal a is required.

Is el prójimo always singular, even though the English translation says 'fellow human beings'?

Yes, el prójimo is a collective noun. It translates to 'neighbor' in the broad, humanitarian sense, but it is almost always treated grammatically as singular in Spanish.