I'm not aware of a way of setting a minimum number of reviewers, but you can set a minimum number of approvals, which by virtue of logic is also a minimum number of reviewers: https://confluence.atlassian.com/bitbucketserver/checks-for-merging-pull-requests-776640039.html
Premier Support Engineer
@Christian Glockner's answer is good and allows the minimum # of approvals to be set per-project or per-repository.
Our own plugin (Control Freak) further enhances this idea to allow the minimum to be defined globally. Control Freak also gives you the ability to not count read-only users towards this threshold (e.g., normally any user can approve any pull-request against a "public" repository - you can use Control Freak to prevent that).
Here's a screenshot from Control Freak's config page:
Hey Community! We’re willing to wager that quite a few of you not only use Bitbucket, but administer it too. Our team is excited to share that we’ll be releasing improvements throughout this month of...
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