You could simply your code and trap NaN entries with
var n1 = Math.floor(Number(form.numb1.value)) || 0; // i.e. assign 0 if the entry is NaN
parseInt() is really intended for converting from one number base to another. If you do
use it you need to specify the radix (10) as otherwise if the user enters (e.g.) 09 it will be interpreted as octal. The correct way to obtain an integer is to use Math.floor(). Another example of poor quality teaching, but if parseInt() is required then I realise you must comply.
Instead of the fixed divisor /5 you need to keep count of the number of numbers entered. Without using an array, you could do that by assigning an initial value of "x" to n1, n2 etc. and then including in the calculation only those variables which are != x.
As Old Pedant says, onkeyup is totally inappropriate. You should use a button to trigger the calculation. I don't see how onchange is relevant either.
To centre (or center in American spelling ) your input boxes:-
“Sex is one of the most wholesome, beautiful and natural experiences that money can buy.” - Steve Martin
<div id = "container" style = "text-align:center"} >
First Number: <input type="text" name="numb1" ><br>
Second Number: <input type="text" name = "numb2" ><br>
Third Number: <input type="text" name="numb3"><br>
Fourth Number: <input type="text" name="numb4"><br>
Fifth Number: <input type="text" name="numb5" ><br>