so will 12 reps be my max rep limit for squats or should i just continue adding reps until i get new plates?
You can keep going but past 12 reps or so it's going to be almost all stamina...but you can go as high as you want. More than 20 is probably a waste unless you really want to work your squat stamina for that level of weight.
Some folks swear by 20-rep sets in squats and deadlifts - Stuart McRobert, who wrote "Brawn" and published Hardgainer magazine was a big fan of 20-rep work for those exercises. But your program has you working 5-7 reps for a reason, so the sooner you can get more weight and get back to where doing 3 x 5 is hard, the better!
i need to bend my wrists in order to hold the bar (which is also illustrated in the book but said that it's the wrong position)
You mean Mark Rippetoe's "Starting Strength" - that's where I got that tape tip too.
You might try to change your grip. I have to hold the bar pretty wide to keep my wrists straight. So much so I have to keep my pinky off the bar because otherwise it might get mashed into the rack. That's because of my big wingspan (I'm 193cm tall and proportioned).
You can also try riding the bar a little higher, if it's down so far it's causing your wrists to bend painfully.
Just a couple suggestions. A recent video showing the problem might help, it's possible you've got something else going on and the wrist is just a symptom.