When you think about it, it doesn't really matter how many ranks there are. If the developers wanted to, they could make each rank worth one point, making there thousands and thousands of ranks. I know that that is the drastic example, but you get my point.
I personally liked 55 levels. I thought the time it took throughout the 20's and 30's took the decent amount of time, and that the 40's, while they were rather tedious back in the time of Call of Duty 4, they seemed to progress reasonably, and same for the 50's.
The additional ten levels presented by World at War, and the additional five brought by Modern Warfare 2 didn't make any significant difference, but they seemed random to some extent, without any reasoning behind them.
Honestly I don't care how many levels there are, so long as there are a reasonable amount, that take a reasonable amount of time to go through. I don't want there to be too few levels, so that I can go through them quickly, but I don't want them to be too many levels, so that it takes me a great deal of time to get through them, as I simply do not have that kind of time to invest into one video game.