No, not one purchase. But yes one merchant. So even if the number is lost and the dollar limit still allows transactions a 3rd party can not charge it.
The details of a temporary CC number vary a lot by the issuing bank. Some services offer numbers that are are truly single use for a single transaction. Some are time limited to 30 days or some such. Some are limited to one merchant (my Discover does this.) Some have a dollar limit. There's also combinations of the above.
I gather that issuing banks are converging on limiting to one merchant and are phasing out other options for that. Remember that the banks are acting in their interest, not the consumers'. A merchant lock keeps you safe from a stolen number, and avoids most fraud scenarios; the banks do care about that since they're legally liable (in US law) for fraudulent charges. But this approach allows the single merchant to make recurring charges (which some customers want protection from); the banks of course have a vested interest in keeping a stream of transactions coming.
