Any hosting that allows unlimited addon domains can host multiple sites. I wont tell you to go get a VPS or Dedi. Start up with a shared host, and expand as you move on.
You can try HG (I wont recommend it for Wordpress though) Justhost or any. I use Justhost for all my use and throw websites. They are like $35 for a month with 24/7 online support and little downtimes. worth the money though.
The extra money you pay for Reseller hosting or a VPS, will be well worth it with the increased control and management, you will save the money over time especially in the future when management of thousands of websites becomes an issue