From the Dreamhost wiki:
Note that the implication is that additional domain registrations have to be paid for, but there is certainly the implication that you can host as many domains as you can "fit" into your one free plan/account.
Back when there was a Strictly Business plan, that was the plan that non-profits got for free. Now that the standard Happy Hosting plan has higher limits that the deluxe Strictly Business plan, the only features that the Strictly Business plan had going for it over the current regular shared hosting plan are:
1. one free unique IP address
2. VPN access
3. five telephone callbacks
4. one free SSL certificate
These are quite valuable and I'd certainly recommend that non-profits contact pre-sales support to see if you still get these with a 501(c)(3) account.
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