Is the 2021 HSA contribution "catch-up" for those over 55 per person (if two are over 55), or per family, no matter how many in the family are over 55?

HSA contributions are individual contributions.  The regular limit for having family-coverage applies to both spouses contributions in aggregate, but the $1,000 catch-up limit applies to each individual separately.  If both spouses are HSA-eligible and one has family HSA coverage, the family limit gets split between contributions to the HSAs of the spouses but each of the spouses who is age 55 or over can separately contribute to their own HSA an additional $1,000 (assuming full-year eligibility).

The catch up applies to each owner of an HSA over 55. 

The family contribution limit may be split any way you choose but the catch-up provision is individual only.  Also note that each HSA account is owned by one person only, there are no joint or marital accounts.

So assuming that both spouses are covered by a family HSA and have no other disqualifying coverage, then the overall limit for 2021 is $9,200.  The first $7,200 can be split any way you like but the $1000 catch up is individual.  So each spouse could contribute $4600, or spouse A could contribute $8200 and spouse B contributes $1000, but neither spouse could contribute $9200 to a single account.