The catch-up provision for people over age 55 is $1,000. This means if you are aged 55 or over, you can add an additional $1,000 toward the annual contribution. This means for 2018 you can contribute up to $4,450 for a single filer and $7,900 for a family if you are 55 or older, with only one HSA.
In the case where both spouses open an HSA, and are both over 55, the annual family maximum contribution could reach $8,900.
Reference:
https://www.medmutual.com/For-Indiv...e-Basics/FAQs-on-Health-Savings-Accounts.aspx
Catch-up contributions: One or both spouses may make catch-up contributions, if age eligible. For example, if both spouses are age 55 or older by the end of 2018,
total contributions to their HSAs cannot exceed $8,900 ($6,900 maximum limit + $1000 catch-up for each spouse), when both have family coverage.
Reference:
https://www.hsaservicecenter.com/hsasc/about.aspx