Domain names should pass what's called a "radio test", meaning if you heard the domain name on the radio would people be confused how to get to it?
The problem with numbers is that it's often not clear whether the number should be spelled out or not. As an extreme example, consider something like "paintball-1source". If you heard that on the radio and the announcer did not spell it out (and they shouldn’t have to) would you know that there's a hyphen or a number 1 in it? Probably not. And even if the announcer spelled it out, how likely would you be to remember it?
There are some notable exceptions, but in general numbers and hyphens cause confusion which is going to reduce the number of people that can find and remember your site.
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