Is one sheep sufficient for the aqiqah of a male newborn when there is no physical ability to slaughter two, or when the price of a sheep for aqiqah is exorbitant due to monopolization, despite the financial ability to purchase it? And is it permissible to substitute a goat for the aqiqah instead?

It is sufficient to offer one sheep (from the domestic sheep or goat species) for the aqeeqah of a child, provided it meets the conditions required for validity in sacrifice, such as being a jadha (a young sheep that has completed six months) from domestic sheep or a thaniyyah (a goat that has completed one year) from goats. By doing so, the fundamental Sunnah is fulfilled, regardless of one's financial or physical ability to offer two sheep or the price of a sheep from the domestic sheep species.