You don't choose a target when you activate the activated ability. You choose a target for the linked triggered ability only if it triggers because the power of Inferno of the Star Mounts became 20.

That triggered ability triggers only if the activated ability causes the power of Inferno of the Star Mounts to become exactly 20. If its power becomes 20 in any other way, the ability won't trigger. If its power becomes 21 or greater this way, the ability won't trigger.

However, if you have a way to decrease its power, you can cause the triggered ability to trigger multiple times in a single turn.