Why two carrier deployments require five or six hulls

Is one Trieste-type carrier enough to cover a distant deployment?

No; three hulls are needed to guarantee one station, and at least five are needed to guarantee two flight decks are available.

Inventory is not the same as deployable strength. Ships rotate through refit and maintenance, so a force of five might routinely provide only two at sea and perhaps a third at short notice. A three-hull group is the rough minimum for guaranteeing one station through those cycles. Guaranteeing one carrier in the Pacific and another near the Falklands therefore requires a much larger pool than two named ships.