How to split a number?

If you want to split one 13 digit number into 4 numbers, then surely Split is the way.

If it's just the way for some validation or input taking then I would suggest you to use a masked textbox to take the input.
Mask TextBox ASP.NET Control[^]
Enhanced Textbox Control[^]

There are few similar control based article here at code project itself.
This sort of control would restrict only given number of digits, no less no more in a specified format. Change it as per your need.

You need to use a validator to enforce your format. Use the string.Split method to turn it into seperate numbers.

for not accepting more than 13 digit number you can use regular expression validator.