Do I need special travel pants?
Under no circumstances are you to pack a second pair of jeans. Pack one pair of jeans. When the jeans rip, throw them out and then buy more. If you are resistant to this idea, ask yourself how long you’re comfortable wearing the same pair of jeans at home.
If you’re constitutionally incapable of going more than two or three days without washing them, then instead of packing more jeans pack one or two extra pairs of the real backpacking MVPs -- of which you need at least one pair -- yoga pants, or whatever athleisure-type equivalent fits your body/style/personality. Basically whatever long pants you’d wear to the gym. They’re eminently more comfortable than jeans, and they’re equally good for outdoor activities and lying around, things for which jeans are suboptimal. They’re also lighter, faster-drying, and more likely to be made of space-age stuff that doesn’t stink. On your trip these are your Regular Pants. Jeans are now Special Pants, to be worn when you’re out getting dinner or drinks.
How many shirts is too many?
It all depends on how comfortable you are wearing dirty ones. But I would say pick maybe three shirts that are comfy, light, and versatile in terms of tan lines and/or aesthetic (meaning you have options for hiking, sleeping, and looking presentable) -- you’ll use all three, and if you end up needing more later, you can buy them later. Unless you’re going to only warm places, you’ll also want one long-sleeved base layer, the temperature-regulating kind you get from activewear places like REI (as opposed to just a long-sleeved shirt). Generally speaking, avoid cotton; you want shit that dries fast and doesn’t smell. Which brings us to:
Travel socks and travel underwear
The next time you need to buy socks, buy a pair of Merino wool socks -- yes, they are more expensive than regular socks, but you don’t need more than two pairs max and they last much longer than whatever you’re wearing right now. They’ll also smell better and be more comfortable. I still use these, because they’re better-made and really do save you money in the long-run (these I have repurchased once or twice though; even miracle socks are still socks, they don’t last indefinitely if you’re wearing them all the time). Oh, this works for Merino-wool underwear, too. Get special underwear. You won’t be sorry.