Addition of numbers using button won't work

Projects Programming
10 
  pinMode(buttonPin, INPUT);

So, tell us HOW the switch AND the external resistor (you MUST have one) are wired.

Or, use INPUT_PULLUP as the mode, and you won't need an external resistor.

   int plus = stock +number;

stock never changes. number never changes. You can push the switch until your finger falls off, and plus will ALWAYS be assigned the same value.