1038
As Dumb Guy points out, it's important to note whether the array starts at zero and is sequential. Since you can make assignments to and unset non-contiguous indices ${#array[@]} is not always the next item at the end of the array.
$ array=(a b c d e f g h)
$ array[42]="i"
$ unset array[2]
$ unset array[3]
$ declare -p array # dump the array so we can see what it contains
declare -a array='([0]="a" [1]="b" [4]="e" [5]="f" [6]="g" [7]="h" [42]="i")'
$ echo ${#array[@]}
7
$ echo ${array[${#array[@]}]}
h
Here's how to get the last index:
$ end=(${!array[@]}) # put all the indices in an array
$ end=${end[@]: -1} # get the last one
$ echo $end
42
That illustrates how to get the last element of an array. You'll often see this:
$ echo ${array[${#array[@]} - 1]}
g
As you can see, because we're dealing with a sparse array, this isn't the last element. This works on both sparse and contiguous arrays, though:
$ echo ${array[@]: -1}
i