If you want an official answer from DreamHost, then you are asking in the wrong place. This forum is primarily a user to user support forum. DreamHost staff do post here on a semi-regular basis, but this is not the best place to get answers from DreamHost. I suggest you use the contact form linked below;
I don't know DreamHost's policies on such things, but I very much doubt any of their shared servers have anywhere near this number of customers on them.
For example, my particular server has around 600 users on it, but since each customer can have many users under their account, the number of customers is going to be less than this. I'd guess the number, for my server at least, would be around 2 or 3 hundred customers.
I will also add that I rarely suffer from any load problems on my server.
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