16 number only

Can I set up a fill in for only 16 numbers? no more and no less?

example: I have account number that must require 16 numbers but some people make a mistake and only fill in 15 sometimes.

I see that you are using Number for the Account Number field in your form. 

May I suggest you use a Textbox instead and set an Input Mask?

https://www.jotform.com/help/381-Getting-Started-with-the-basics-of-Input-Masking

This way, your respondents will be forced to enter 16 digits for the account number field. An error will be displayed if they fail to do so.

I hope this helps. Let us know if you need any further assistance. Thank you.