How to select 8 bits?
You need a 2 digit BCD (Binary Coded Decimal) to binary converter. Here's a way to do it using 4 bit adders:-

 

Alternatively you might consider leaving the inputs in BCD, and processing them directly using a BCD adder and BCD counter.

If you intend to actually build this circuit then you could also eliminate the BCD encoder logic and reduce the number of physical switches by using BCD rotary switches.