It does not matter how many tests new software has already completed, or how well it performed during those trials, when a new app is almost ready to launch the only way to know for certain it will perform as expected is load testing. There are tools available for this job that come highly recommended for the success of your new "baby." These tools simulate demands on software and enable one to see how it will endure in different situations. Another, perhaps even tougher way to find out what software can really take is stress testing. This method intentionally over-loads software until it breaks by applying situations not likely to occur in normal use.
Of course, use of the word "normal" could and often does mean many things when it comes to technology. Each method is invaluable in determining exactly how much software can actually handle in the real world being used by real people. It should be noted that if software truly isn't ready for future demands, it is common for a load test to morph into a stress test while running. The terminology can often be confusing when one program acts like another and the best way to avoid it is simple research. Know all the facts before you ever launch a test to avoid unpleasant surprises or delays.
It is easy to miss a few details when developing or testing code, and it's when multiple users all try the same command at once that those "misses" are likely to crop up causing your program to crash. Like testing any new product piece by piece, there is no way to really know until they are all fitted together how things are going to behave. Since the ultimate goal of load or stress testing is to determine functionality, keep end-user's from losing their minds, and protect a businesses reputation and/or bottom line the thing to do is figure out which one of those three is most important. Obviously, that is going to be the person online trying to make use of the software, since it is their happiness with a product that is going to matter most in the end.