MaxClients is the number of simultaneous clients that a server can process. If you are using the prefork MPM there is one process to one client, and for the worker MPM, there is one thread to one client.
While the request itself might be brief, you still have to consider keep alive requests where one clent can tie up a process for more than one request. You also have to remember that not all users have 100mbps connections to the Internet, so if it takes a user 20 seconds to download an image, one of your MaxClients is tied-up until the user finishes.
Whether 150 process is too many depends on what you are doing and what your hardware is like, but even a moderately priced server gets you 16 cpu threads --so those 150 processes end up being around 10/cpu thread.