If you had a look at the task manager for all users, then you might have seen lot of processes running under the name svchost.exe. To define, svchost.exe is the process for services that are contained within dynamically-linked libraries (DLLs). Windows runs on many DLL file and since these DLLs cant be run directly, svchost.exe is used to execute them. Since many services run under Windows, you are seeing so many svchost.exe in Windows.
The main issue with so many svchost.exe files running is that malwares and rogue programs can also disguise themselves as svchost.exe and run under Windows. But you can always check which all services are running under a particular svchost.exe by using the task manager. Right click on any svchost.exe file in task manager and select the option Go to Services.
This will open the services tab and select all the services which the svchost.exe is using. Here you can see if the services running under the svchost.exe is needed or not.
HowtoGeek has an excellent article on how to check the services running under an svchost.exe file. Alternatively you can also download the svchost viewer which can give complete details on svchost.exe file. In case you feel that some services are not needed, you can stop them using the service manager in Windows 7 and in turn stop the svchost.exe file associated with the service.