Depends on area where You live and substrate You use and size of water dish You Have... I'll start with 5 on front and back and 7 each side... and then just monitor your environment...
In My rack I have 15 on front 8 on back and around 19 holes on sides ... with aspen sanichips and 8 inches water dish i have constant humidity around 55-60%...
But that is in rack with free tubs I'll go with less than that....