Here's what I take on 3 week outings in late summer: 1 shorts, 1 long pants, 2 T-shirts, 1 long sleeve bug off shirt, 3 pair wool socks, 1 pair merino top and bottom, 1 fleecy jacket, 1 rain jacket, 1 pair boots, 1 camp sandal.  Since my trips in later Fall are typically under 5 days, I take pretty much the same thing but throw in an extra fleecy.  Anyway, as to the shirts.  I've completely gotten rid of anything cotton from my kit - too slow drying, too heavy and a disaster ecologically.  So for T-shirts I recommend Bamboo (actually 70% bamboo, 30% cotton.)  They are silky soft, durable, don't stink, dry much faster than cotton and are kinder to the planet.  They are comparably priced to cotton T's.  Here's where I get mine but that's because they are located here in Toronto.
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But if I were getting a dedicated camp shirt I'd probably get a synthetic bug off type shirt.  They have them at Sierra Trading Post all the time marked way down.  
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I have an Ex Officio shirt with insect shield that is just about the best shirt I've ever had for camping - they're light and vented so that even in the heat and direct sun you can wear it and stay fairly cool plus they're great going through buggy ports or sitting around camp at sunset when the skeeters start a hummin.