On your NetBenefits home page, look for the words "Health savings account": your 9-digit HSA number is directly underneath.
On the NetBenefits summary page for your health savings account, your 9-digit HSA number is at the top of the page.
On your Portfolio Summary page on Fidelity.com, in the left-hand column, your 9-digit HSA number appears below the words "Health savings account."
If you access your accounts on your mobile device, go to the listing of your accounts and look for your HSA: your number will be listed.