All DreamHost plans come with ‘unlimited’ domains and sub-domains, so you should not have too many problems in this regard.
Here on DreamHost, each domain or sub-domain has its own web accessable directory, which makes them mostly independent of each other. This is different, and in my opinion vastly superior, to some other systems that configure sub-domains as sub-directories of existing domains.
In short, DreamHost is extremely well suited to hosting lots of domains.
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