The buggy feature helps you keep track of your buggies but it will allow the over booking of buggies. However, the system will display a red message at the top of the screen if they have been over booked:
Overbooked Buggy Warning
To remove this message, the number of over booked buggies will need to be removed from the day's timesheet or if you have other buggies available to book, these should be added to the buggy count via Tools > System Configuration.