I don't want to be facetious but if that works and it's the same sort of content and usage then three times what you have now is probably a good starting point. Moodle scales readily by throwing more hardware at it.
If you want to give us a description of your current architecture then we can comment how easily that will scale and any other considerations.
If you want to give us a description of your current architecture then we can comment how easily that will scale and any other considerations.