I'm glad you've reached out to us. Allow me to help and provide some insights about processing last payroll of the year.
In this case, whether your payroll includes hours between 2018 and 2019, you only need to create 1 paycheck since the tax liability of your payroll depends on its pay date and not the period in which work was performed.
You can check this article to learn more about the Constructive Receipt principle in QuickBooks Desktop Payroll.
For future reference, you can also check this article: Payroll 101.
Please know that you're always welcome to post in QuickBooks Community if you have any other concerns about Payroll. We're always here to help you out.