Yesterday my wife was out of town. I had food on the fridge, but I thought I needed to cheer up my Monday, so I rented a movie and ordered pizza. At some point my wife called by the phone, and I was scratching my dog's belly, watching the movie, and paying attention to the porch, because I was hungry and expecting the pizza deliverer to appear at some point, all while talking to her. If we decompose anything one can do into that kind of mechanics of keeping concentration (that, if I recall, are for maintaining concentration spells going on) with these +3 steps, I was EF 21 concentration rolls without noticing!
I don't think the point isn't how many Prying Eyes spells you can maintain at once. I'm not sure how much attention you pay to your TVs, but I can certainly keep it turnt on and be somehow aware of what's going on, noticing when I have to focus on it. Re-reading the spell description the part that seems to require some effort and to concentrate is to change the viewpoint. So, yes, casting a handful of Prying Eyes as the spell comes out of the books probably deserves that 21+ EF concentration roll. But any variation of fixed duration, let's say Diameter or Sun, would just require the caster to focus to change his viewpoint. I wouldn't ever dare to require for a concentration roll, unless in extreme, stressful circumstances, like in combat or engaged in complex lab activity. Also I wouldn't start making analogies as "set of security tv monitors inside your brain", because thought it helps us making an idea of how to interpret the spell results, it obviates the fact that this is magic inside a game of magic. Don't you remember replying "it works because it's magic" back when you were children?
Anyway, Prying Eyes sucks. Any busybody magus will at some point come with something like:
Prying Watch of the Busibody Bored Magus, InIn 35, R: Arc., D: Moon, T: Structure
As Prying Eyes, with modified parameters, so you don't need to concentrate to maintain the spell, nor go around touching walls or limit yourself to one room. You might change your viewpoint within the structure by concentrating.
(Base 1, +4 Arcane, +3 Moon, +3 Structure)
Or even better! Do you want to know if dinner's ready at the king's kitchen, how it smells, if it's salty or dull, and the roast is well done? What's her maid saying in the basement and to whom? If the rider that just arrived have his coat wet and cold? Get some details regarding a quite intimately placed mole to blackmail the queen as she takes her seasonal bath? This is for you:
Prying Watch of the Busibody, Bored and Quite Possible Lascive Magus, InIn 50, R: Arc., D: Sun, T: Structure
As above but working for all senses (and cut down to Sun duration, because rituals).
(Base 5, +4 Arcane, +2 Sun, +3 Structure)