int vs. int64: Which one should I use?

yourbasic.org/golang

An int is either 32 or 64 bits depending on the platform. The length of an array can always be represented by an int.

In this example the data has type int64, while the index and the length of the slice has type int.

func Reverse(a []int64) {
	n := len(a)
	for i := 0; i < n/2; i++ {
		a[i], a[n-i-1] = a[n-i-1], a[i]
	}
}

Here is an example from the time package.

// A Duration represents the elapsed time between two instants
// as an int64 nanosecond count. The representation limits the
// largest representable duration to approximately 290 years.
type Duration int64

The Maximum value of an int article shows how to compute the size and limit values of an int as untyped constants.