according to my idea we can do only one project by using one Arduino board (uno,mega,)
Arduinos are quite flexible as a development board and platform,
however as with all development boards there are certain restrictions dictated by the design.
Designing a project from the ground up always offers more flexibility at the cost of convenience.
I'm little bit familiar with pic i did some projects using it
When you design a project using a PIC, is the entire design compromised of only a PIC?
Of course not!
You must design a complete circuit, including a power supply, oscillator section, sensor input, various outputs and possibly a programming interface.
The Arduino is simply a prepackaged design, commonly known as a development board, including all the common design requirements listed above.
Rather than use an Arduino, you could purchase a PIC development board or design and build your own PIC development board.
what is the best thing remain in pic or jump to Arduino
You can only answer that question, as it largely depends on your intended goals.
As I mentioned before, the Arduino is a good educational platform, it provides a standardized design, IDE and programming interface.
However, it is quite a bit different than traditional embedded C development as it mixes many elements of C++ with Standard C.
Also the convenience of the standardized Arduino design comes at a cost of flexibility.
Another option is studying the Atmel AVR microcontroller and later building your own Arduino out of an AVR microcontroller.
The above option would offer you advantages of both low level development and the higher level development using Arduino IDE.
BigDog