How many of the visitors should move from shared hosting to a dedicated server?

the criterion of "number of visitors" in this matter I think not the primary.
The main criteria is performance (when Your project starts to slow down on shared hosting, or creates a serious burden because that starts the problems of other clients and You simply are asked to leave), price (large amounts of traffic from a dedicated server are often more profitable than shared hosting), security and reliability (sometimes for greater reliability and security, and for projects with dozens of visits per day from a dedicated server to take).

loadimpact.com/
will help you to understand
good luck (:

the choice is simple: when you a day of downtime costs approximately the cost of your hosting for the year, you will not hesitate to buy your server or arenduet it

Yes, as you begin to kick, then move on.
The problem is that the same units of measurement of the load on the virtual hosting is not.
And no one can predict load as a function of the number of visitors.

So maybe easier to put home server on Intel Atom and stick it in ETTH with an outbound channel is 4 Mbit/s?

the service is at least described, OS, PL, DB... and yet the answer is that as soon as braking starts and start to think.

Why not consider an interim solution — a VPS/VDS? On the one hand, many of the disadvantages of "simple" hosting (e.g., the inability to control or fine tuning) are on the other is much less true Dedic