From the player's perspective, zero checks would be optimal since automatic success is better than leaving your fate to a d20. Per the rules, there are three tasks to perform - finding it, figuring out how it works, and then disabling it. If the player can remove the uncertainty as to the outcome and/or the meaningful consequence for failure from a task, then there is no check. So in practice, there could be anywhere from 0 to 3 checks.
That's consistent with how you've described your approach elsewhere. I'm wondering how one removes the consequences for failure regarding a trap? (Or was that more a statement of the general principle?) Genuinely curious.
We'd have to have details of the trap to be able to decide whether something a character does (be it finding it, figuring it out, or disabling it) succeeds without a check. Note as well that the other condition for avoiding checks is removing uncertainty as to outcome. I can find a glyph of warding with a detect magic spell, for example, removing uncertainty as to the outcome of finding it. I can throw a rope around a chest protected by this glyph and drag it 10 feet away to dispel it, which makes disabling it automatically successful (provided there's not some trigger on the glyph that says otherwise).
Thanks. Hasn't come up much in the campaigns I'm running, and it's good to know how someone who handles it a lot (which IIRC you do) handles it.
I think, in general, players have been trained to just want to roll their two checks or whatever without really interacting with the trap. "I search for traps. Okay, now I disable it." This is boring in my view and really takes the value out of exploration challenges like traps, plus it's actually not very smart play on the part of the player. That second task of figuring out how the trap works is a trigger for players to come up with creative ideas on how to disable it without necessarily leaving the d20 to decide. Skip that step in the process and you leave all those interesting ideas on the table.
See PHB, page 178, "Finding a Hidden Object" and DMG, pages 120-121, "Traps in Play." Also DMG, page 103, "Secret Doors."
As both sets of rules mention finding a hidden object, and neither are more specific than general as they both mention the exact same thing - determining if there is, and where that item is - making two checks to do the exact same thing neuters characters.
