How many domains/addresses can I host in a single plan?

With the exception of the Starter plan. we don't limit the number of domains or addresses in our email plans. You can add as many as you like, as long as you stay within your plan's quotas. As long as you have space, go for it. Just remember that you're responsible for your overall quota, so don't add more than you can handle, or all of your addresses will become unusable if you go over your allotted storage quota.