Intervention can be unilateral or coordinated between several authorities, and it can be announced, denied or left ambiguous. Selling reserves to support your own currency is limited by how much you hold, which is why fx-reserves figures are watched; buying your own currency down is theoretically unlimited, because you can create it.
The tactics vary. Some authorities act in size at a sensitive moment to force stops and change the trend; others trickle through agent banks to slow a move. The market's response depends less on the amount than on whether traders believe the policy behind it, which is why an intervention that fights the interest-rate-differential rarely holds for long.
History supplies both outcomes. Japan has repeatedly bought yen against the dollar in the modern era, with roughly ¥9 trillion disclosed across September and October 2022. The Swiss National Bank's defence of a ceiling ended differently, as described in swiss-franc-unpeg.
Example: a bank sells $10 billion of reserves to buy its currency. If the pair drops 2% and then retraces fully over a fortnight, the reserves are spent and the rate is where it started.
Related: sterilised-intervention, verbal-intervention, fx-reserves, currency-peg