Ok, let's start with the what here.
If you want to make a wizard encounter into a wand, great. The one you're looking at is a L8 magic wand. That's a +2 wand, great from L4 up through oh say about L10 or so. But there's a problem, there's not really any information as to how this works exactly. The only way I know of for a PC to craft a magic item is via the Enchant Magic Items ritual. This requires that the caster of the ritual be L8 and it costs as much as a L8 magic item (the wands can be updated from there, but that doesn't serve our purpose much).
Since they have errata'd the magic item daily powers, there is nothing stopping you from using several of them per day. There is no corresponding errata preventing you from creating magic items of unknown rarity, so stuffing a wizard encounter into a dozen wands seems legitimately possible provided you have the cash. In theory, you can indeed create an unlimited number of these wands (provided you have enough money). However, that's not really all that useful.
- You have to be a wizard or wizard MC with at least one wizard power swap feat (though wording indicates simply an arcane MC may be enough, so perhaps Bard, but you still need access to the power to do the creation, so you'll need a wizard friend perhaps at least). At this point, you might as well just take a power swap feat to get Shield itself. It's an encounter, which is probably how often you'd bother using it.
- It's expensive and doesn't scale without more investment. A L8 implement isn't cheap (specially not for a L8 character, it gets cheaper as you level, but if you want to use these as implements it stays expensive and you have to keep upgrading it). Keep in mind that the power doesn't scale at all, just the enhancement bonus.
- You have better things to spend your immediate reaction on. To be honest, Shield is a great spell at L2, it can be a nice get out of a single hit power for your whole career, but by the time you're high enough level to pull this off, you should have things that actually let you do damage with your immediate action every round. If you're a wizard, the odds of you getting attacked often enough to make this strategy worth it are nearly nil. If you're a melee character, well, most of those classes thrive on out of turn attacks, this would be a waste of resources both monetarily and action economy-wise.
So yes, nothing stops you from doing this. But I don't really see the point.
No, you only get 1 immediate action per round so even if you have 12 wands of Shield and could hot swap them, you could only use 1/round.