Cloud / Server
In Atlassian Cloud, the maximum licensed user count is 5000 users (Confluence, JIRA Core, JIRA Software).
For JIRA Service Desk Cloud, there is no limit on the number of 'customers'. For licensed users (in JIRA Service Desk this would be the "agents" role) you can have up to 5000 licensed agents.
For Server, there is no limit on either customer (free users in JIRA Service Desk) or on licensed users. Check out the pricing points for all user tiers including the 250+ agents (i.e. unlimited in JIRA Service Desk) as well as the 10,000+ (also unlimited in Confluence and JIRA Software/Core) here: