Is it obligatory to say "Amen" at the end of every supplication, or is it sufficient to say it once at the end of the entire supplication? And is it better to say "Allahumma, Amen, for You are All-Hearing, All-Responsive" or "Our Lord, accept from us, for You are All-Hearing, All-Responsive"?

Saying "Ameen" after a supplication (Du'a) is recommended, not obligatory. It is particularly emphasized in prayer after reciting Al-Fatiha, as the Prophet (peace be upon him) said: "When he says: 'not of those upon whom is wrath, nor of those who are astray' – then say: 'Ameen,' and Allah will answer you."

There is no explicit text from the scholars regarding the specific practice of saying "Ameen" after every single supplication or after concluding all supplications. The apparent understanding is that the matter is broad. Phrases like "Ameen," "Ameen, O Lord," and others are all virtuous, and their meaning is to request the acceptance and answering of the supplication. And Allah knows best.