Are there any requirements/guidelines for an Android device? like numbers of buttons or minimum buttons required.
Also are there any android devices which do not have the menu and back buttons?
( I am aware that no menu/back buttons will kill most of the apps in terms of usability , I just wanted to know more on the topic :-) )
Are there any requirements/guidelines for an Android device? like numbers of buttons or minimum buttons required.
Yes. These are documented in the Compatibility Definition Document.
Also are there any android devices which do not have the menu and back buttons?
That depends on how you define "buttons" and "android devices". Quoting from the CDD (see above link):
The Home, Menu and Back functions are essential to the Android navigation paradigm. Device implementations MUST make these functions available to the user at all times, regardless of application state. These functions SHOULD be implemented via dedicated buttons. They MAY be implemented using software, gestures, touch panel, etc., but if so they MUST be always accessible and not obscure or interfere with the available application display area.
There are devices that do not have dedicated off-screen buttons (whether physical or touch-sensitive off-screen spots). The ARCHOS 5 Android tablet is one -- it has the HOME and BACK buttons in an expanded title bar. However, it is unclear if Google considered them to have met the CDD, since the ARCHOS does not have the Android Market. Devices lacking the Market may not meet the CDD.
So, it is entirely possible to create devices that do not meet the CDD, but at that point Android is mostly just another embedded OS, IMHO.