those servers on the mainchain don't need to handle thousands of Blockchain apps, because every blockchain app will run on its own sidechain with its own delegates. That's the beauty of the concept with the sidechains. The mainchain delegates need to handle only the traffic and transactions on the mainchain, which is basically sendind LSK, voting, and registering sidechains. All the traffic from apps are handled on their own sidechain by other delegates (depends on the creator of the app how his consensus works and who are the delegates, if any)
Will there be different numbers of delegates connected to an blockchain app? So does that mean, blockchain app 'A' will have 50 delagates, blockchain app 'B' will have 10 delagates blockchain app 'C' will have 160 delagates? Who decide which blockchain app gets how many delegates?
How many delegates are on each sidechain is determined by the app developers. So, yes. It can and most likely will vary.
In theory, you could run Ethereum on a Lisk sidechain.