How can I deauthorize computers if I don't have any of them anymore

Use the Deauthorize All command documented in this article, or if it isn't an option, the 'contact iTunes Store support' link at the bottom of it.

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You need to deauthorize all computers and then reauthorize the ones you still have. See the quote below from Deauthorize your computer using iTunes - Apple Support

My computer doesn't work anymore. Can I deauthorize it?

You can't deauthorize a single computer if you don't have it or can't use it. Instead, you need to deauthorize all computers and then reauthorize the ones you're still using.