On the text field configuration, under advanced section, you can set the Max Length field.    This will restrict the number of characters one can type, assuming you mean characters and not "numbers" specifically.

Thanks

Ok, that is because you have the text field "connected" to a column in the list.   In this case you need to go to List Settings, and click on the column name you want to set a max character limit for, and enter this number in the Max number of characters field in the list itself.  The Nintex form will respect the number you set here.

Thanks

You can use a validation with Regular expression:

Example if you only need 7 numbers to be entered in the field, double click on the field for settings.  Then Select Yes for "Use Regular Expression" and enter the expression  .{7}

Thx!

In the event that you want numbers only, instead of a period ".", please use the following for 7 digits only... ^d{7}$