The total value of all HSA accounts is the sum of what you have in the account(s) on a given day (12/31/2018, I assume).
Note that if you are being asked this question, you probably have an excess contribution that you are not withdrawing. The IRS charges you the lesser of 6% on excess contributions that you carry over or the total value of your HSA.
The total value of each HSA (you probably have only one) can be found on form 5498-SA. If you have not received this form yet, go to your HSA administrator's website and see if you can download it from there, or alternatively, speak to a customer service agent at that website.
