ermm, lemme see... (5x2)-1...If u want authentic button positions, than ya, u can only have 9 floors but if u use the back wall for the button area... 30 floors. But rly, it would take so long to program the elevator for more than thirty floors that even with more space the whole thing would become obsolete.
Edit: I looked at your WTC build, It's an empty shell! no wonder its 70 floors! but if u did want to make an elevator, u could just make three different shafts. (as in, 1-30, 30-60, 60-70)