Ooh! Great question! The answer is...it depends.
I keep my rotation far from my cellar. There's a floor and a half between my rotation, my open tins and jars and pipes, and my cellar in the basement. And 4/5 of my cellar is bulk that's been jarred up. A couple times a year I'll go through my rotation and swap out, put this in the cellar and bring that up. And whenever I get a new order in, of course.
And real quick, I want to mention some of you fine fellas have some beautiful tobacco storage arrangements. If I ever had all my tobacco in one area like that, I wouldn't have a single solitary unopened tin or jar. I'm convinced of that.
So for open tins, I'm counting tins and jars I'm smoking currently. My choices when deciding what I want to smoke. I try to keep that to a dozen. I'm at 15 right now, but I can explain. 4 of those are work blends. I had 4 jars or tins to smoke at work. Mostly dry nicotine heavy (or heavier) or well behaved in the pipe and pleasant smelling. I'm between jobs, so I'll take my 4 out of daily home rotation and I'll be at 11. That's room for that extra one that we all end up with somehow.
But wait! There's more!
In my car I have 3 jars for driving smokes. And 4 in my wife's car. I have a pouch of 5 Brothers in my fishing tackle box along with a cob. I have a dried out pouch of SWRA and a cob in my shotgun case (I should probably switch that out for a pouch of Captain Black or Sutliff).
I suppose these are all open tins and jars. In court a lawyer could probably argue I have 24 open tins, pouches, or jars. But I don't count those last 9 as in my rotation. They're separate, like corn cob pipes don't get counted in my pipes. Everyone's milage varies differently, and this is how Rookie math works, and the Internal Revenue Service can suck it!