Android apps runs on top of Java. Each application creates its own virtual machine. Initially when android OS boots up a virtual machine 'zygote' is created, which is then forked to create every new process in the system.
With increase in the number of processes on the a system. More system resources are consumed, CPU will be more busy, context switching is increased, but this is how modern system works.
Dalvik VM, which is equivalent of JVM, is loaded for each running process. If an Android phone user has 30 apps, which is common, there is a big chance that these apps are running almost all the time. Some apps creates 'services' in android which runs in the background. For example, Facebook, google maps etc.
Even if a user has closed an app these apps keep on running in the background to provide notifications and real time updates.
Running service and apps in an Android Phone can be viewed in
Settings ->
Applications
-> Running Services
As a user keep on downloading and installing applications, chances are that these apps will be running most of the time in the form of 'services'. More apps running simultaneously will consume more CPU even if it is Quad-Core. More CPU consumption means the Phone will be more power hungry and battery consumption will also increased.
Applications like Facebook creates multiple services on Android phones. Services in android can be run as a thread in a process or it can create a separate process, but this depends on the developer of the application.
A good practice can be to only keep those apps which are useful, all other apps should be deleted to improve the phone experience.
