Technically you can host as many servers as you want on any server. It is just the matter of load that the server is able to withstand. A stupid old celeron server cannot handle 1000 dynamic sites while it will handle upto 2500 static sites with ease. When your requirement for processing increases, you naturally need to upgrade your server.

Certain datacentres support upgrade of motherboards and processors alone, and you can retain the same IP and the hard drives that you would have been using earlier, while other datacentres ask you to do a complete migration.

When your demand for load increases, it is advisable that you go for the best server that is available in the market, and put it as many processors as possible onto one server, stick in as many hard drives as possible, and use it. Linux can handle multile hard drives easily, while Windows control panels cannot. You will have to use Raid.

But when you have all sites on one server, which is a lot difficult to manage than having them on different servers, how do you manage the load? Even if one site overloads the database engine, all sites hosted on the servers could go down. So why not use multiple servers? Well, I ask, why not virtualize your monster server itself?

Get a VPS license for about 10 nodes, and virtualize your server, and split them into 10 induvidual servers, and allocate resources accordingly and split low resource consuming sites, dynamic sites and overloading sites seperately.

The cost of operation will naturally be a bit high, but compared to the quality of service that you provide to your clients, and the cost of maintaining individual servers seperately, this idea is a lot better. And, you have all under one roof, and the admin can access all files and folders with his root acess.

This is a good idea for all linux servers, while I cannot recommend it for windows. Infact, I dont recommend anything for Windows at all.

Trust me, its the most difficult job in the world. You can transfer cargo, just put them in containers and load them on ships and unload them where ever you want, but backing up with plesk from the old server and moving it to the new server and restoring it there is the most difficult thing that I have ever done.

Personally I have shifted homes, I have shifted offices, shifted my own websites between servers before I started a hosting company, and now I find that all very easy. No matter how many men you have to do the work, you cant work any faster than the server would. Plesk is dead slow, and with all those sites working on the server, and with clients from all over the world, there is no off peek time for your server, you will never make it.

Plesk has a backup restore function, which you need to use from the remote desktop. Create the backup selecting resellers or domains in each task, and once its complete, move the backup file to the new server, and restore it there thru the plesk bakcup resotre function.

Sounds easy? It will, till you see it. I had 12 people working on it for 24 hours a day for nearly 20 days last year to get 1 server migrated. And in the last leg of the migration, the hard disk failed on the old server, and unfortunately, the backup files that we had created were yet to be moved to the new server, and all data was lost.

Just google with indyahozting and you will find out the bad reviews that we had got because of that. But still, we were quick enough to get the new server ready and ask people to upload the backups that they have had. Luckily most of the clients had their backups handy, and they restored all files, except for the few ones, who just decided to write about us on the forums.