I want to set a minimum number of characters

You never specified to which field you want this option so I'll just point you to the Short Text Entry (or textbox) field. You can find a LIMIT ENTRY option under the OPTIONS tab of its properties window.

In the example above, turning ON the LIMIT ENTRY option and setting its value to 5 would mean users can only type in 5 characters on that field.

Is this perhaps related to the "Employee #" field on this form?

https://www.jotform.com/62655054084961

I'm asking because it looks like you're expecting "numbers" to be entered on that field. With this in mind, you can use the NUMBER field instead of the SHORT TEXT ENTRY field.

The NUMBER field allows both MINIMUM and MAXIMUM limit entries.

Another alternative is to use INPUT MASKING on the SHORT TEXT ENTRY field.

Complete guide: Getting-Started-with-the-Basics-of-Input-Masking