Answer
As a general rule, the default checkout names will increment by one to the next available unique number for the checkout. For example:
MyCheckOut
MyCheckOut_1
MyCheckOut_2
MyCheckOut_3
However, if a version already exists with a name that matches what would be the next available checkout name, that name/number will be skipped and the next number will be used instead to ensure the combination of checkout name and version name will be unique when the checkout is created.