Well, you should take a look at the forums here first off.
1. Is there always some business in this type of thing?
Yeah, just like with most businesses. The challenge is staying in business
2. I will have to use Cpanel, but how can I do for the Vps?
You should keep the 2 seperate.
3. How many sites can I host on this very server?
It seems the average site is 500mb, some will scale and so of course you will never know for sure. Your only focus should be making a profit and growing your business. It will be different for every server you have.
I would like to sell some shared, reseller and vps hosting.
If you have no idea on how to manage a system or how processes work, my suggestion would be to learn before you even start otherwise you are only going to get sick and tired and then dump it.
Just my professional opinion, it happens all the time.
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If you want my opinion, its sounds to me like you are way in over your head. If I were you, I would simply cancel the dedicated server and sign-up for a reseller account with a reputable provider so you can learn the ropes. It will save you money and lots of headaches, especially if you are not experienced in properly managing a server (which it sounds like to me, like you are not).