I have an 8.5x8.5 album that is 3 years of Christmases and I counted 37 page protectors. I wouldn't recommend trying to squeeze 50 page protectors in one this size--if your 50 pages=25 protectors then it might be ok. I do need to replace my album because the posts pulled through one in one place.
The only thing would be the number of photos you have. At 4x6 you can only get 2 to a (crowded) page unless you do some cropping.