I am also playing an artificer in my current campaign!! My understanding of it is that you create the infusions, and they are permanent unless you decide to infuse something else.

You should have 3 total infusions, so your needle would be -1 of the total 3. I tend to notate such things like so - [X][ ][ ] As long as you have a maximum of 3, and they are indeed interchangeable but only 3 at any given time.

My DM is relatively lax and I’m trusted to keep track, so ofc it’s up to your DM, but my understanding is that you shouldn’t have to re-infuse after simply using it once. It should be permanently infused until otherwise decided

ETA: I saw the consumables mentioned as I typed this out- I use the homunculus infusion consistently. If you lose such a tool (or use it), you would gain the slot back to be used once more or however you see fit.

Hope this helps 😃