In order to limit the number of months on the booking form from property page you must add maxDate in functions check_in_out_enable  and check_in_out_enable2 from  wprentals\js\control.js file

Help: http://api.jqueryui.com/datepicker/#option-maxDate

add  maxDate: ‘+3M’, as displayed below

Change number of months from 3 to number of months you need.