Since these are all brand-new and well-honed blades, I'd say yes to the N12k now. That should be all you need for touch-ups. It's a surprisingly fast stone for its grit size.

As far as working on practice blades, limit your honing on them to the 8k level until you're getting a smooth shaving edge... then they'll respond to your 12k.

Since you've got this wealth of nice brand-new blades, I'd recommend that you keep one of them out of your rotation--hold it back as a reference blade. The edges degrade so slowly that it's like an eyeglass prescription... you don't realize the degradation is happening until the day you get a new pair of glasses with the new prescription. Oh yeah, that's how well it's possible to see! Oh yeah, that's how sharp a razor should be!

This will be especially helpful because your technique will be improving at the same time your blades are dulling. Every 6 weeks, maybe, pull out your reference blade and just do one swipe under a sideburn. If it feels significantly better than your other new blades, try touching them up on pasted felt or the 12k.

6 weeks might be too soon for a hone touchup, or it might not be... depends on your stropping technique.

Good luck with it all. You're off to a well equipped start. Here's a temptation to fight, at least until you get some junkers to practice on: Just because you HAVE all those hones doesn't mean you need to USE all those hones.

Best wishes to you.