You ask about 'Turns Ratio'.
The 'Turns Ratio is the same as the 'Voltage Ratio'. So, if you want to transform 120 Volt to 24 Volt, then you will need a turns ratio of 120/24= 5.0
However, as crutshow says, the REQUIRED number of turns on the primary side cannot be guessed because it depends on the factors crutshow mentions. The power required to be transformed affects the core size because the wire gauge needs to be thicker for a higher power transformer. The design of these things is somewhat cut and try, because of the restricted number of core sizes available.
When the number of turns is decided for the primary, then only one fifth of that number of turns will be needed on the 24 volt side. As tvtech says, the secondary wire diameter will be thicker than the primary wire by the ratio square root 5