With Aref at 5V (default setting) the analog readings will come back in the range of 0 to 1023, with each step representing ~4.88mV.
1023/15 = 68, 68 * 4.88mV = 0.332V.
A string of 15 1K resistors should work pretty well, adjust the analog read break points to match:
Vout = 5v*1000/(1000+14000) = 0.3125V
Vout = 5V * 2000/(2000+13000) = 0.667
Vout = 5V * 3000/(3000+12000) = 1V
Vout = 5V * 4000/(4000+11000) = 1.33V
etc.
val = analogRead(Ax);
if ( (val >=0) && (val <=67){
// button 0
}
if ( (val >=68) && (val <=135){
// button 1
}
if ( (val >=136) && (val <=203){
// button 2
}
// etc
