I currently have 5 which I maintain everyday and about 20 plus for my clients registered under my name because they trust me so much.
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Currently owning 3, it gets very confusing if you're owning more.
But they're on auto renew each year so I don't have to worry about them unless I'm adding a sub domain.
Ever since 2000 I have owned more than 100 domains. I sold a few which I was offered a good deal. Having such huge hoard of domains I couldn’t manage all while others were not quite productive hence I left a few dozens to expire. So far I only have a few are standing strong. Up to now I only need ten domains that can stand on their own hence I am planning to sell a dozen. However I don’t market them which have caused me to take more than four months now looking for a buyer.