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This question already has an answer here:

If I own a domain (www.mydomain.com), what are the theoretical and practical limits on the number of sub-domains I can create (eg meta.mydomain.com, beta.mydomain.com)?

marked as duplicate by Simon Hayter, Stephen Ostermiller, John Conde Mar 13 '13 at 2:56

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I think it's unlimited. For example, you can see

When you create an account, you have your pseudo as a subdomain. And as you may know, Tumblr & Wordpress have a lot of members!

I found some interesting articles:

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