Box 5 of my csa 1099-r only has one number in it, but TT is asking how much of it is health insurance. Is there something I'm missing?

On for CSA-1099-R Box 5 can contain a combination of Employee Contributions, Designated ROTH Contributions or Insurance Premiums.

If you have an amount in Box 5 the program is asking if any of that amount is for Health Insurance that TurboTax should add to medical expenses to be deducted on Schedule A.

How do I find out how much was for insurance. 

It is usually a notation on the form.  If you do not see a notation then the amount is usually $0, but if unsure you will need to ask the issuer of the form to clarify.