If you read between the lines in the Instructions for form 8889 for line 6 (see the examples for step 4), if you can legally be covered by the ex-spouse's HDHP policy and the policy is Family coverage and you do not have any conflicting coverage (like Medicare or a "normal" health policy), then you are eligible to open your own HSA account and contribute the Family maximum to it of $7,200 for 2021 (up to $8,200 if you are 55 or older), without regard to what your ex contributed to his HSA.
Note, however, that this requires you to maintain some level of communication with your ex. The moment you stop being covered by the HDHP policy (it might not occur to him to mention changes of his policy to you immediately), you need to recalculate your annual HSA contribution limit based on the number of months you were actually covered (coverage on the first day of the month determines coverage for the month).
Also, this presume that the HDHP policy that he has is a Family policy (covers him and at least one other person, whether you or a dependent). If he were to change it to a Self-only policy, then your limit for 2021 would be $3,600 (or up to $4,600 if you are 55 or older).
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