Will that be able to track the achievement stat for every chest opened?You don't use Steam's API for that. What you gotta do is activate the achievement as usual, and then go back to the typical RGSS3 API and turn a switch on. Don't forget to add that switch as a condition for any treasure chest event to open those chests for you.
Is it some one time achievement? Then you'd have to reverse the order of the steps I mentioned above.
I don't recall any incremental type of achievement on Steam so I doubt this interface includes such a thing at all.
That would work well, but I'd prefer to have Steam track a stat so players know how many chests they are missing for the achievement.If I may say, why not just make a variable that goes up every time you open a chest, then have a common event running that checks to see if the variable is equal to the number of chests, and then when it does, trigger the achievement?