First, merge all your separate JIRA Server instances into one - see https://confluence.atlassian.com/jirakb/how-to-merge-jira-applications-instances-244352137.html
Then migrate the result to Cloud - see https://confluence.atlassian.com/jirakb/migrating-from-jira-server-to-jira-cloud-779160826.html
Pretty much the same. Export from Cloud, get that loaded into the latest version of Server, then merge in all the other server systems (this is not usually a "fun" job, I'm doing it for a client at the moment, and it's really finnicky. You'll find it easier due to some bug fixes in JIRA 7)
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