The server hosting market is separated into single, double, qual, etc., processors, where each processor has several cores, or CPUs.

My company will offer a Linux-based web application that relies on an Apache web server and a middle tier for business logic. The middle tier is used to crunch math, and return result to a client. Many clients may access the application simultaneously. The company will start with one processor having 4 cores. I'm trying to understand how the app uses the cores and then how to scale the application as business grows, in terms of servers/processors/cores.

For example, I'd assume initially one core would be used for Apache, and the other 3 used to process client's requests for math crunching...

Question 1: does that mean, with the 3 cores available, I can handle 3 separate client requests simultaneously (e.g. 1 for each of 3 cores)? I mean, except for the shared RAM, is this effectively like having 3 individual machines (from pt of view or processing client requests simulaneously)? Or, only one client's request may be processed at any one time, but that client's request is divided up into up to 3 cores depending on the type of process running that does the math crunching and whether or not it can take advantage of multi threading (so the # of cores impacts how fast any one client request completes)? I'm confused about what the cores mean to the application here.

Question 2: As the business grows and more client requests need to be processed, should the server be upgraded to (A) a new machine with more cores, (B) a new machine with two processors, 4 cores each, or (C) keep the original server and add another server with a single processor? Which route provides the most efficient way to scale the application, in terms of processing more client requests per time interval? Is the choice, for example, limited by RAM (when you need more RAM than box can handle it's time to add another server), or something else?

Question 3: Is the total number of client requests processed simultaneously equal to the number of cores times the number of servers (minus the one core for Apache)?