There are so many things to factor in, it is still not a very fair question.
It is unfair to answer about buttons. There is a differnce between maximum wanted for a reasonable playing experiance and maximum that you can shove on a panel so all your possible button needs would be covered.
It is unfair to answer anything about joysticks, that is a matter of preference. What some people love, other people hate. Even Joysticks that are commonly accepted as being junk, you will find that some people would not use anything else.
It is unfair getting into trackballs and spinners, there are a few games that take like 3 trackballs, we could tell you that you would need 3 trackballs, but then others would say not to because thats overkill. Many of us cannot honestly answer your question of what is the maximum, because we would never dare put all that on our machines.
It is unfair getting into other specialty controls, you can spend you life savings on specialty conrtols to play about anything under the sun, but we cant just tell you to get 8 steering wheels, a perscope, two lightguns, yolks, handlebars, a dancepad, a majjongg panel...etc. etc. However, there is some things that most of us highly reccomend that you get if you are all about trying to play as much as possible.
I have to shake my head at myself, because my previous posted advice has been as confusing as everybody elses.

It's not our fault, we are just trying to help in our own way.
You should probably start with a search, I am almost definite what you really want to know has been answered a half dozen times previously on the board. Also, if you want to ask a question, try to stay away from maximum/minimum. We are a lot better at answering questions if you fit it to your needs, rather than the largest possible amount.
Believe it or not, we are usually quite helpful, you questions just need to be phrased a little easier for us to understand, we are all trying to read between the lines of what you really want.
Hope this helps clear up why your answers are not so black and white.

Vigo